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Fish Pond Filtration

Fish ponds present unique filtration requirements that are easily solved once you understand the basics.

By Stephen M. Meyer

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Gardeners spend their winter months poring through seed catalogs in anticipation of spring. This is part of the fun of gardening. For pondkeepers, winter is also a time for planning. Pond filtration is too important and costly an endevor to be approached in a haphazard manner. If you are thinking about building a pond or improving an existing pond, now is the time to plan and prepare for that day in spring when you will actually begin your project.

I receive many letters through Aquarium Fish Intl. that ultimately boil down to questions of pond filtration. Most ornamental fish ponds — just like conventional home aquariums — are 100-percent recirculating systems: the same water remains in the pond all the time unless some special effort is made to change it. The levels of biological pollutants in the water rise quickly unless the pond is in ecological balance or a supplementary filtration system is used to slow the rate of decline in water quality.

When setting up a home aquarium, you can easily purchase simple, effective mechanical, chemical and biological filtration systems that do a good job of maintaining water quality. Even the first-time hobbyist recognizes the need for some aquarium filtration, and store shelves are loaded with an amazing variety of filters and accessories.

In contrast, filtration is almost always an after thought for the budding pond enthusiast. The pond is built, fish and plants are dumped in and the pond is done. Things are usually fine for a while — longer than with standard aquariums because there is much more water involved. Eventually, however, water quality-related problems usually do arise.

Part of the trouble is that many first-time pondkeepers are initially hooked by the idea of having their own miniature version of a "natural pond." The notion of creating an ecologically balanced system of fish, plants and insects is indeed attractive, and there are many fine examples where it actually does work. All too often, however, natural pond enthusiasts quickly violate the rules of ecological balance by stocking their ponds with quantities of fish out of all proportion to a properly balanced ratio. Or, a pond that was initially in balance with many small, young fish is pushed out of balance within a few years as the fish grow rapidly to maturity. For example, it might seem reasonable to stock a small pond with several one-year-old koi. A typical year-old koi could weigh 1.4 ounces or so. This same fish could weigh more than 14 ounces by the time it is three years old. Therefore, even if the number of fish in the pond remains the same, the total fish load in the pond will increase 10-fold within several years.

When a pondkeeper inquires about filters, he or she learns that the assortment of commercial mechanical, chemical and biological filter systems readily available to aquarists has no counterpart in the world of ponds. While there are numerous so-called "complete" pond filtration systems available for purchase, they vary significantly in their practical value, effectiveness and price. To wisely choose among them, you need to know a considerable amount about filtration principles and techniques. Some pondkeepers, frustrated by the lack of commercially available filtration systems to choose from, realize that they might be better off attempting to design and build their own pond filter. It is certainly less expensive and almost always more effective when tailored to a particular pond.

This article looks at the fundamentals of pond filtration, with an emphasis on practical design and operation. More than 20 years of research and experience by fisheries scientists, aquaculturalists and waste water treatment specialists has produced a substantial body of findings that is directly relevant to the filtration of ornamental ponds. There are many possible filter designs that adhere to these principles. I really don't believe that there is any single optimum filter system design — although some may be better than others in terms of operation and maintenance. Consequently, this article steers clear of proposing a particular design.

You may be wondering why you even need a pond filter. As I have already noted, most ornamental ponds are 100-percent recirculating systems. Except for rainfall or water changes by the pondkeeper, no new water is added to the pond. Under these circumstances, water quality declines fairly quickly, and the pondkeeper must contend with consequent problems of fish health and pond aesthetics. A properly designed filtration system can significantly slow the decline of water quality, although no filter can maintain the pristine conditions of an open aquatic system, such as a spring-fed pond.

There are three basic types of pond pollutants: nitrogenous wastes, suspended and settled solids and dissolved organic carbon. The most serious water quality problem for the pondkeeper, by far, is control of nitrogenous wastes — in particular, ammonia. Unfortunately, you cannot have fish in your pond without also having ammonia constantly being added to the water. Fish continuously excrete ammonia through their gills, and additional ammonia is released into the pond water by the decay of fish feces and uneaten food. Moreover, almost any organic matter — solid or dissolved — likely to fall into your pond can be partially mineralized into ammonia.

Ammonia appears to hinder oxygen uptake in the fish's blood. Lethal concentrations can build up rapidly in heavily stocked ponds. Continuous lower concentrations of ammonia cause chronic physical stress that ultimately reduces a fish's ability to fight disease. Gill filaments become irritated and begin to swell. This cuts off the oxygen supply to the membrane cells, which then become infected with bacteria. Kidney damage has also been observed under these conditions. Low concentration, ammonia-related stress also reduces food consumption and growth, inhibits reproduction and causes premature death.

Bacterial consumption of ammonia produces toxic levels of another nitrogen compound, nitrite. Nitrite, like ammonia, interferes with the ability of the fish's blood to transport oxygen. Other bacteria convert nitrite to nitrate, which is much less toxic to fish. Contrary to popular belief, the nitrate levels commonly found in ornamental ponds do not affect goldfish and koi in any measurable way. Toxic effects do not show up until levels reach several hundred parts per million (ppm).

Any materials that do not dissolve into the water are considered to be solids, which can vary from microscopic particulates, such as tree pollen, to large objects, such as leaf and tree fragments. Suspended solids are undissolved materials that stay afloat in the water (though not necessarily on the surface), circulating around the pond. In an ornamental pond, suspended solids may be composed of fine mineral particles that flow into the pond via rain, wind, ground runoff and so on, detritus (a composite of finely ground-up organic and inorganic material originating from plants and animals) and plankton, the microscopic plants and animals that dwell in the pond.

The greater the quantity of suspended solids in the water, the higher the turbidity. One of the most common causes of high turbidity in ponds is planktonic algae, which turns the water pea soup green. From an aesthetic standpoint, not only does turbidity reduce transparency through the water, but it also makes the pond look dirty. In addition, some forms of suspended solids can also irritate fish gills, causing stress and making the fish more susceptible to disease. Fancy goldfish seem to be especially sensitive to turbid waters, whereas koi and pool comets (a variety of goldfish) appear to be totally unaffected.

Settled solids drift to the sides and bottom of the pond. Unless disturbed, they tend to sit where they land. Koi keepers know, however, that these fish constantly stir up settled matter, adding to the turbidity of the water. Admittedly, the distinction between suspended and settled solids is somewhat artificial, because under very quiet conditions and with enough time, many suspended solids will settle out. Then again, many settled solids can be resuspended by stirring up the water. In either case, even though these solids ultimately leave suspension, they affect the aesthetic and biological qualities of the pond.

Suspended and settled solids are composed largely of organic matter, and thus are susceptible to decay. Bacteria in the water will convert (mineralize) some of this matter into ammonia, which then adds to the overall nitrogenous waste burden in the pond.

Another water quality concern is dissolved organic carbon (DOC). DOC consists of carbon-based compounds (excluding carbon dioxide and carbonates) that are the metabolic by-products of pond life. Carbohydrates, proteins, amino acids, fats, phenolic compounds and pheromones (hormones that affect the behavior of other fish) are just a few examples. Concentrations of DOC may be as low as 1 ppm in clean rivers and lakes, whereas polluted waterways may have levels up to 100 ppm.

The bacterial decomposition (mineralization) of some forms of DOC will add directly to the ammonia load in the pond. DOC can affect fish health by supporting populations of disease-causing (pathogenic) bacteria and parasites. It can also have a direct biological affect on fish when pheromones are present, inhibiting growth and lowering disease resistance. What is important in this latter case is the origin and composition of the DOC, not necessarily the total DOC concentration. On the one hand, in ponds with very light fish loads, the primary source of DOC will be the metabolic products of plants and planktonic algae. Cellulose-decomposing bacteria will dominate the water's bacterial population, and only very low concentrations of fish growth and immune system-inhibiting DOC will be in the water. On the other hand, in heavily loaded ponds, the fish are the main source of DOC, and the composition of the DOC is quite different from lightly loaded ponds. This DOC will support rich populations of "protein-eating" bacteria and fungi, some of which can cause disease in fish. The much greater concentration of fish-originated DOC will directly suppress fish growth and immunity, making the animals more susceptible to bacterial and parasitic infections. DOC pollution is of greatest concern when the primary source of DOC is the fish load.

The implications of all this are clear: your pond must have some mechanism for controlling the relative concentrations of solids, DOC and nitrogenous wastes if your fish are going to live and thrive. Some ponds can handle these pollutants without any supplementary filtration, whereas other ponds require substantial assistance.

The question then becomes whether or not you need a pond filter. The answer will depend on the kind of pond you plan to manage and how you manage it. Let me draw a useful distinction between a garden pond and a fish pond. A garden pond is an attempt to recreate an ecologically balanced system among plants and fish. The key here is that the amount and variety of plants are high, while the quantity of fish is very low. I have seen many gorgeous examples of garden ponds. They vary in size, but all have the same common situation — the plants are the primary attraction and the fish are almost secondary.

A typical garden pond might be kidney bean shaped — 8 feet long, 5 feet wide and 2 feet deep — and hold about 550 gallons. Plantings would include several waterlilies, a variety of submerged plants, such as Anacharis, Cabomba and Myriophyllum, floating plants, such as water lettuce or water hyacinth, and assorted bog plants, such as iris and horsetail that rise out of the water. Fish in the pond would be limited to perhaps a pair or two of goldfish or several small koi.

This garden pond depends on ecological balance instead of filtration to maintain water quality. The fish load is intentionally kept very low so that the pollutants they produce can be handled by the bacteria, plants and microscopic flora and fauna that populate the pond. Most of the solids settle out of the water — resulting in low turbidity — and a considerable amount is broken down by bacteria in the pond. Of course, an occasional hand skimming of leaves and other debris helps maintain low solids levels. Concentrations of the DOC we are most concerned about remain low because 1) the fish load is low, 2) mineralization removes some DOC and 3) partial water changes remove some DOC. Lastly, the level of nitrogenous wastes produced in the pond remains low (because the fish load is low) and is rendered harmless by the activity of nitrifying bacteria living on the pond walls and plants. The nitrate produced by nitrification is consumed by the plants. No supplementary filtration is required.

In order to maintain this balance, no additional fish are added. As the fish grow, the fish load is kept constant by judicious culling. If spawning occurs, the new arrivals are quickly removed so as not to upset the balance.

In contrast, a fish pond is dedicated to the breeding and showing of fish. Its primary purpose is to raise fish for display, as is the case with most ornamental koi ponds. Here, the fish dominate the plants, and often there are no plants at all. Fish loads are quite high — far in excess of what you would ever find in nature.

There is simply no way to achieve an ecological balance in such cases, and therefore substantial supplementary filtration is needed. The solids, DOC and nitrogenous wastes are produced in quantities far in excess of what the population of bacteria in the pond can handle. Moreover, most of the solids and DOC are of fish origin, increasing the importance of removing them. They will just accumulate over time until the fish begin to get sick and die — tragically bringing the pond back into balance.

Because pond conditions vary so widely — much more so than conventional home aquariums — pond filtration needs vary as well. Some ponds may only have a problem with the accumulation of solids and turbidity, whereas others may only need a means of removing nitrogenous wastes. More than anything else, the fish load will determine how you should assemble your pond filtration system. Below, we will discuss the component parts of supplementary filtration for fish ponds, examining various alternatives for removing solids and DOC. In part two, we will consider methods for controlling nitrogenous wastes.

Reducing solids requires some type of mechanical filtration that physically removes solids from the pond water. There are two basic approaches worth considering: gravitational settling and physical screening.

Gravitational settling is a process by which solids denser than water eventual drift to the sides and bottom of the pond under the force of gravity. Pond turbidity frequently increases in the aftermath of a heavy rain storm as materials are washed into the water and previously settled materials are stirred up. Subsequent settling activity usually leads to substantial clearing in a few days. Essentially, any pond is a settling basin. But rather than allowing this matter to settle anywhere in the pond, it is better to either direct settling to occur in a specific part of the pond or to have settling occur in a special basin outside the pond to facilitate removal.

In the first instance, a special depression — a sump — is built into the pond floor. Over time, settlable solids drift down into this area, where a bottom drain, a siphon or a pump is used to remove this material. Although a sump is simple, it has two problems: 1) a sump cannot be added after the pond is built and 2) koi and goldfish love to muck around in sumps, stirring up debris.

A preferable alternative is to add a settling basin outside the pond. The settling basin is a transitory waterway through which pond water is pumped. It is big enough so that solids settle out before the water returns to the pond. The advantages of a settling basin are that it can be added to a pond system at any time, it is not accessible to the fish and draining and flushing are simple.

How big should the settling basin be? All else being equal, the time required for a given particle to settle out depends on its density, shape and size. Dense, small, round particles settle out faster than less dense, larger, flat particles. In general, for our 100-percent recirculating pond, the larger the settling basin volume is in relation to the pond volume, the greater the amount of solids that will settle out.

Taken too far, this could get quite absurd, with the settling basin approaching the size of the pond! So, let me suggest a more reasonable rule of thumb. What little research has been published suggests that settling times of 15 minutes or more are required to remove heavier wastes. Thus, we want the pond water to spend at least 15 minutes moving through the settling basin before it goes on to the next part of the filter system and ultimately returns to the pond. For reasons that will be explained in part two of this article, we also want to pump one complete pond volume through the filter system at least every two hours. These two criteria suggest that the settling basin volume should be roughly 1/8th the total pond volume. For instance, a 1000-gallon pond should have a settling basin that holds about 125 gallons.

To improve operating effectiveness, the basin should be about 2 feet deep and slope upwards toward the end where the water exits. A drain valve should be placed at the deep end for daily solids removal. The effectiveness of settling basins can be improved further by the intermittent placing of objects in the basin to restrict the flow of water. Brushes, fiber mats or any other form of partial barrier can be used to slow the velocity of suspended solids in the water, forcing them to settle out. In this way, smaller basins can serve larger ponds.

An advanced form of gravitational settling — one more appropriate for intensive fish farming and rearing ponds with very high fish loads — involves the use of "hydroclones" and centrifuges. These devices accelerate pond water around a circular route at rates many times that of gravity. Thus, the separation of solids is more rapid and effective compared with ordinary gravitational separation in sumps and basins. Because accelerated gravitational separation is so much more effective, a hydroclone or centrifuge system can be much smaller than a standard settling basin, and may be worth considering in situations where space constraints prevent construction of a simple settling basin.

A hydroclone for a typical ornamental pond might be a cone 1 or 2 feet in diameter, tapering down to 3 inches as it reaches a depth of several feet. Water enters at the top of the hydroclone and is directed along the cone's outer wall at high velocity. Solid wastes settle to the outer wall, drift downward and drop out through the sludge outlet at the bottom. Clean water exits from a pipe placed about 1 foot below the surface at the center of the cone. Commercial units are available, although they are quite expensive.

The alternative to settling is screening. For our purposes, a screen can be any porous barrier with a fixed mesh size that is placed in the flow of water. Coarse screens — such as a grid of ½-inch PVC pipe spaced ¼-inch apart, or egg crate lighting grills — can be used to removed leaves and other large debris. Window screening stretched across a frame will trap particles down a few millimeters in diameter. Thin foam sheets and nylon floss held between two egg crate lighting grills can provide very fine mechanical filtration. Cleaning involves daily washing of the screens, and intermittent replacement.

Even better mechanical filtration can be obtained with a somewhat different "screen:" a gravity sand and gravel filter. In this design, water flows over a layered bed of sand and gravel and then trickles down between the grains. The scrubbed water exits at the bottom of the bed, leaving the solids behind. Gravity sand filters require daily backwashing to keep the bed from clogging.

Although the quest for ever finer mechanical filtration might seem worthwhile, keep in mind that the better the screen is at removing solids, the more frequent will be the cleaning requirements. At the extreme, diatomaceous earth filters certainly offer the best capability for removing ultrasmall particles, but they are totally impractical for pond filtration. They are so effective that they literally require hourly backflushing and cleaning.

The do-it-yourselfer has a wide variety of options for building a mechanical screen filter. The size, mesh size and number of stages can be varied. Personal experience suggests that you will get more mechanical filtration and fewer maintenance headaches if you keep your filter screens thin but give them a large surface area.

There are several commercial alternatives available as well. Danner Manufacturing (160 Oval Dr., Central Islip, NY 11722) sells a thin foam screen wrapped around a plastic cylinder under the product name Supreme Poolmaster Universal. This filter is very effective for small ponds (under 500 gallons), but several can be ganged together for larger ponds.

Another option — one that is most suited for large ponds over several thousand gallons — is the standard swimming pool pressurized sand filter. Using a coarse sand (1 mm in diameter) or even #3 aquarium gravel, pressurized sand filters are very efficient mechanical scrubbers. Although a pressurized sand filter is easy to install and operate, there are a number of drawbacks to these units. First, the pumps used with rapid sand filters consume lots of electricity and are very noisy. Unless you have some remote place to house the filter, the noise can ruin the pond setting. Second, daily backwashing means high water consumption — a couple of hundred gallons per day. Third, because the sand bed is quite deep, over time it tends to clog to the point where backflushing is useless. Once this happens, the filter sand must be replaced.

Keep in mind that the solids removed by a mechanical filter of any design remain in contact with the pond water until they are flushed from the filter. They are still "available" for bacterial decomposition that will degrade water quality. Therefore, as I have already noted, you should flush the mechanical filter at least once each day. Less frequent cleaning means that the results of mechanical filtration will be largely cosmetic.

Of course, it is quite feasible to combine a settling basin with mechanical screening. Designs are open to the imagination. You could, for instance, start with a settling basin with a water course of brushes, followed by a coarse mesh screen, and end with a fine sand bed.

We now come to the topic of removing DOC. DOC removal in standard freshwater aquariums is commonly accomplished with the use of granular activated carbon (GAC). GAC, however, is quickly fouled by particulate matter, planktonic algae and other substances that abound in the average ornamental pond, but which are much more scarce in aquariums. A 1000-gallon pond might require 8.8 pounds of GAC or more each month! Thus, GAC is not a practical approach to DOC control in ornamental ponds unless you are quite wealthy.

Frequent partial water changes will, of course, keep DOC levels down. But this can get very expensive with large ponds, and many parts of the country are experiencing water shortages that limit such activity. Moreover, if your fish pond is heavily stocked, nothing short of a 50-percent water change every day would matter.

An alternative method for removing DOC is the foam fractionator (more commonly known as a protein skimmer). This is a simple device that uses a rising column of bubbles to extract the DOC from the water. In fact, you have probably seen foam fractionation at work in your pond without realizing it. The appearance of a white scummy froth at the base of a waterfall is caused by bubbles coated with DOC.

Foam fractionation works by adsorption, taking advantage of the fact that many DOC molecules have a polar structure, with one end that is attracted to water and one end that is repelled by it. The repelled end will attach itself to the surface of air bubbles rising through a column of water, and when the bubble is removed from the water the DOC molecule goes with it.

Foam fractionators for hobbyist pond use are not commonly available commercially, but they are quite simple to build using ordinary PVC pipe Water is pumped into the foam fractionator through a standard spa jet. A valve placed before the spa jet controls the water flow, and here again the flow should allow at least one pond volume turnover every two hours. A constriction in the throat of the jet — called a venturi — causes a pressure drop as water moves through the nozzle. An air tube is centered above the venturi, and air is drawn into the water flow at the location of the pressure drop. The air-water mixture moves up the fractionator column and adsorption occurs. The DOC-enriched foam collects at the top of the column and ultimately builds up to the point where it spills out through the exit port. The water moves back down the column in a counter-current flow and is stripped of additional DOC before passing down and around to the water exit column.

You can operate the foam fractionator on its own pump, independent of the main pond pump, or you can hook it in series or in parallel to the main pond pump. The most important thing is to move approximately one pond volume through the foam fractionator every two hours. This means that large ponds could require several units ganged together.

Depending on the conditions in your pond, it may take a few hours or a few days before your foam fractionator begins to produce results. Do not be surprised if the foam you get is not as thick and rich as the kind that bubbles out of marine aquarium foam fractionators. Freshwater systems tend to produce "thinner" foams. You can adjust the foam by playing with the three regulating mechanisms until the consistency is as thick as will flow smoothly.

In the second half of this article, I will discuss the design criteria for the last component of our supplementary pond filtration system: the biological filter for detoxifying nitrogenous wastes. The final task will be to combine the three elements together.

Building a Pond Foam Fractionator
The main body of the pond foam fractionator consists of two columns attached at the bottom by a 180-degree elbow. The larger column is the main column (A) where DOC adsorption occurs. The smaller column — the water exit column (K) — stabilizes the water level in the main column and allows clean water to exit.

The main column (A) is constructed of 4-inch PVC pipe. It should be roughly 8 inches long. The top is fitted with a 4-inch-to-2-inch reducer (B). A small section of 2-inch PVC pipe (C) — which will vary in length depending up your particular situation — connects to a 2-inch-to-1-inch reducer (D). Lastly, a section of 1-inch flex-hose (E), which serves as the foam exit port, is attached to the top and directed away from the pond.

The bottom end of the main column is fitted with a 4-inch by 2-inch sanitary T-fitting (F). The 2-inch opening will house the water inlet spa jet (G). The air tube (H) for the spa jet must extend above the water level in the main column.

Below the sanitary T-fitting is a 2-inch-long section of 4-inch pipe, followed by a 4-inch-to-3-inch reducer (I). Next comes the 180-degree turn, composed of two 90-degree elbows (J). The water exit column is made of 3-inch PVC pipe (K). It is about 10 inches long and is fitted with a 90-degree elbow (L) on top.

This foam fractionator uses three regulating mechanisms. First, as already mentioned, the water exit column (K) controls the height of the water in the main column. Second, a valve (M) is placed before the water inlet to regulate the venturi effect in the spa jet (G). Third, the actual length of the small section of 2-inch PVC pipe (C) on the top of the main column can be adjusted to alter the consistency of the foam.

If you place the foam fractionator in the pond or settling basin, there is no need to cement this unit together. You certainly do not want to cement the top section, which may require adjustment over time.

All the parts you will need to build a pond foam fractionator are available at your local plumbing supply store, except the spa jet. Spa jets can be purchased at any pool and spa retail outlet. The last item you will need is a pump (or you can tap off your main filter pump).

Ammonia and nitrite poisoning probably account for more pond fish deaths than any other single cause. There is really no reason for this, because the mechanisms of ammonia and nitrite poisoning and the methods for avoiding them are well understood. The simple fact is that once your pond is established — that is, after the first few months — and if you are managing things properly, there should never be any measurable concentrations of ammonia or nitrite in the water.

As I described in the first part of this article, it is useful to distinguish between a garden pond — one dominated by plants, where fish are a minor component — and a fish pond designed specifically to raise and display fish under high biological load conditions. In a garden pond, the quantity of ammonia produced each day by a few fish and by bacterial decomposition of organic matter is quite small compared to the volume of water. This small amount of nitrogenous waste is easily rendered harmless by nitrifying bacteria that live on the pond walls, plant stems and so on. Thus, as long as the fish load remains low and organic matter is not allowed to overwhelm the pond, the average concentration of ammonia will remain immeasurable and harmless.

To illustrate these points, let's take the example of keeping three orandas in a 400-gallon garden pond. We can estimate the approximate amount of ammonia they produce each day in one of several ways (see sidebar to the right).

Using the information in the sidebar, let's calculate the daily ammonia load. Each of the orandas probably weighs about 60 grams (just over 2 ounces). Assuming that they are healthy fish, at normal water temperatures each fish will produce about 15 milligrams of ammonia per day (25 milligrams of ammonia divided by 100 grams of weight times 60). The daily total ammonia production for all three fish is therefore 45 milligrams. Dissolved in a pond of about 1515 liters (400 gallons times 3.785), this would amount to 0.03 parts per million (ppm) of ammonia per day (45 divided by 1515). Keep in mind that 1 milligram per liter is the same as 1 ppm. If the pond is kept fairly clean of rotting organic material, ammonia from other sources will be insignificant and the nitrifying bacteria in the pond will be able to handle the daily ammonia levels without any problem. Because ammonia test kits for hobbyists do not register ammonia levels below 0.1 ppm, you would never obtain a measurable reading.

In a fish pond, however, the quantity of ammonia produced each day dwarfs the capacity of the pond-dwelling bacteria to detoxify the ammonia. As a result, toxic ammonia concentrations build up and the fish begin to weaken and die. Let's use a 1200-gallon fish pond with 42 koi averaging 10 inches in length as an example. Koi of this length may each weigh about 190 grams (6.7 ounces). Therefore, they each would produce around 47.5 milligrams of ammonia per day, totaling about 1995 milligrams per day for all the fish. This amount, dissolved in 1200 gallons (4540 liters) of water, would add about 0.44 ppm of ammonia to the water each day. In other words, the daily ammonia load produced in this fish pond is more than 10 times that of the garden pond noted above. Inasmuch as the population of nitrifying bacteria in the fish pond cannot manage this amount of ammonia each day, the concentration of ammonia will accumulate quickly to toxic levels, possibly exceeding 2 ppm after a week. In this pond many of the koi would become sick and die.

Because the natural biological processes present in a garden pond are not sufficient to detoxify ammonia in a fish pond, the only alternative is to set up a supplementary filtration system to do the job. There are two options for the pondkeeper: chemical filtration using ion-exchange or biological filtration. In my opinion, the former is poorly suited for continuous use in the hobbyist pond environment and therefore, for reasons I will describe, is best avoided.

Estimating The Daily Ammonia Load

It is often useful to estimate the daily ammonia load in a pond or holding tank. Fortunately, there are two fairly straightforward methods for doing this.

Unless a pond is totally mismanaged, the primary source of ammonia is the fish. On average, healthy koi and goldfish will produce about 25 milligrams of ammonia for every 100 grams of body mass. Thus, a 12-inch koi weighing 300 grams (10.6 ounces) will produce about 75 milligrams of ammonia per day. To determine the mass of your fish, you can weigh your fish on a scale or you can place each fish in a tub or container graduated in milliliters. Read the volume of water and then remove the fish — either by hand or with a net — without removing any water. Now read the volume again, which will be less than the first reading. The difference between the first and second readings is the volume of the fish in milliliters. The average density of a fish is 1 gram per milliliter. Thus, if your fish displaced a volume of 400 milliliters, it has a mass of about 400 grams, or 0.4 kilograms. By the way, this technique will work with any species of fish.

If determining the individual mass of your fish is too difficult, there is another option. First, using a kitchen gram scale, weigh the amount of food you offer your pond fish each day. The weight should be recorded in grams. If the protein content of the food is between 30 and 40 percent (standard goldfish and koi pellets), multiply the total weight by 25. (If the protein content is substantially higher than 40 percent, multiply by 30, and if it is significantly less than 30 percent, multiply by 20.) This gives a good estimate of daily ammonia production in milligrams per day, assuming that the food you feed is the primary diet of the fish. For example, if your fish pellets are 36 protein and you feed about 30 grams per day to the fish in your pond, the quantity of ammonia produced in the pond each day will be about 750 milligrams.

Ammonia Removal Via Ion Exchange
Ion exchange is a chemical filtration process that removes the ionized form of ammonia (ammonium) from pond water by swapping it for a different chemical ion in the ion exchange medium. The two forms of ammonia, ionized and un-ionized, are explained in the sidebar to the right. The ion exchanger removes the un-ionized, and far more toxic, form of ammonia — free ammonia — indirectly. Because the proportions of the two forms remains constant at a given pH and temperature, removing one from the water causes some of the other to convert.

In ponds, natural zeolites (which look like little chips of cement) can be used as an ion-exchange medium for ammonia removal. The amount of zeolite you will need depends on the ammonia production rate in the pond and the pond volume. Zeolites also vary considerably in their adsorption capacity. A safe average estimate is to assume that approximately 1 milligram of ammonium can be adsorbed by 1 gram of zeolite. Accordingly, the fish pond described above would need 1995 grams — or almost 4.4 pounds! — of zeolite to handle just a single day's ammonia production.

In order to use zeolite for ammonia removal, pond water must be pumped through the zeolite medium at least once every two hours. I have visited ponds in which bags of zeolite were just sitting in the pond water, providing almost no filtering effect. The hydraulic load — the volume of water pumped through the zeolite per unit of surface area — necessary to provide adequate ammonia removal should be between 1 and 2 gallons per minute per square foot of zeolite medium.

Natural zeolites can be "recharged" in heavy salt solutions. This eliminates the need to buy dozens of pounds of fresh zeolite every week when the medium becomes saturated with ammonia. When a 10-percent salt solution that is several times the volume of the zeolite medium is trickled through the zeolite bed for 24 hours, approximately 75 percent of the zeolite's ammonia-removing capacity is reestablished. (Theoretically, zeolite could be 90 percent recharged by this process. In a pond setup, however, organic contaminants usually block some fraction of the medium's surface, preventing complete recharging.) For this reason, it is best to design a zeolite filter with at least 25 percent additional capacity. Therefore, in the fish pond we used as an example, this would mean increasing the zeolite quantity to 5.5 pounds to adsorb one day's worth of ammonia.

Unfortunately, there are several drawbacks to using zeolite in ponds. First and foremost, even moderate levels of particulate matter and dissolved organic carbon in the water significantly reduce ammonia ion exchange. Thus, the effective use of zeolite requires continuous and extensive mechanical and chemical filtration of particulates and dissolved organics. This is just not realistic in the average hobbyist pond that employs 100 percent recycling of water.

Second, if the zeolite medium is left in the pond filter for more than a week, a bacterial and algal film develops on the surface and severely blocks ammonia adsorption. More importantly, the zeolite bed can become a source of nitrite poisoning as nitrifying bacteria establish themselves on the bed and begin to convert the trapped ammonia into nitrite. Because the zeolite is removed and replaced every week or two, the bacteria that would ordinarily consume the nitrite do not have an opportunity to become established. If left in the filter for 30 days, the zeolite is actually transformed into a biological filter — but all ion-exchange ceases.

Third, because the addition of salt to the water will cause the zeolite to release the ammonia it has trapped, it is not possible to use salt for medicinal purposes in ponds with zeolite filters. Although you could remove the zeolite before adding salt, there would no longer be an ammonia removal system and the fish would suffer even more as ammonia levels increased.

Fourth, over time you are likely to add new fish to the pond, and the fish you already have will continue to grow. This means the daily ammonia load will be continually increasing. The zeolite filter, however, cannot adjust, and what was once a sufficient quantity of zeolite medium for perhaps seven days will soon shrink to five days, then three days and so on.

Lastly, there is the problem of what to do with all of the saltwater used to recharge the zeolite. You cannot pour it into the garden or onto the lawn, yet it must be disposed of.

In short, if want to use zeolite for ammonia control, you should not allow the zeolite to remain in the filter for more than a week. It should then be removed for recharging and replaced by an equal volume of recharged zeolite. For our fish pond this would mean swapping 38.5 pounds of zeolite weekly. Moreover, it is necessary to test the water for ammonia and nitrite every couple of days to make sure that the zeolite is still adsorbing ammonia. In my opinion, this suggests that while zeolite may be useful for temporary ammonia control in ponds and holding tanks, it is not the process of choice for continual use by pondkeepers.

Biological Filtration
A far simpler and more reliable process for the continuous control of ammonia in ponds is biological filtration. Biological filtration actually detoxifies ammonia using two types of nitrifying bacteria to do the work. The first bacteria convert the ammonia to nitrite, after which the second bacteria convert the nitrite to comparatively nontoxic nitrate. This process is known as nitrification or the nitrogen cycle. You need do little more than provide the bacteria with hospitable surfaces to grow on and make sure that a continuous flow of pond water loaded with ammonia and oxygen is pumped through the filter.

Ammonia And Nitrite Toxicity

There should never be a measurable quantity of ammonia in a properly managed pond. In other words, total ammonia should always be below 0.1 ppm. Nevertheless, there are times when ammonia levels might rise temporarily and it is good to know when real problems are likely to start in the pond.

Ammonia in water comes in two forms: ionized ammonium and un-ionized (free) ammonia. This distinction is important because it is the latter that is most toxic to fish. I recommend that average levels of free ammonia never exceed 0.005 ppm.

At a given pH and temperature, the relative amounts of these two forms of ammonia stay in constant proportion. As pH goes up, or as temperature increases, the relative proportion of free ammonia increases. For example, in a freshwater pond with a pH of 7.0 and a temperature of 77 degrees Fahrenheit, the percentage of free ammonia is 0.55 percent, with ammonium comprising 99.45 percent. However, as pH and temperature change, so do the relative concentrations of the two forms of ammonia. At a pH of 8.0, free ammonia makes up 5.28 percent of the total ammonia, while ammonium drops to 94.72 percent. In both cases, if you were to remove one form of the ammonia, some of the other form would convert in order to maintain the proportional relationship.

Table I show the limits of safe levels of total ammonia (the ammonia measured with a test kit) in your pond as a function of pH and temperature. These total ammonia limits correspond to a maximum free ammonia concentration of 0.005 ppm.

Similarly, there should never be any measurable nitrite in your pond. Whenever nitrite levels begin to creep up above 0.3 ppm, you should consider water changes to reduce the concentration. Adding salt to your pond at a rate of 1 pound per 120 gallons will protect your fish from nitrite poisoning.

Indeed, a biological filter is little more than a water-proof box that holds the medium on which the bacteria grow. There are many ways to construct such a filter. Some people build them out of concrete, others use trash pails. There are two kinds — submerged and trickle biological filters. In a submerged filter the filter medium is always under water, whereas in a trickle filter the water passes across the medium. There are, in fact, hundreds of variations of these basic designs. Most work well. In the limited amount of space here, let me offer some general design principles that are common to the best filter systems.

There are many variables that determine the optimum characteristics of a biological filter for a given pond setup. Fortunately, they can be reduced to a few key considerations in order to give you a good approximation of what your biological filter should look like.

The most important consideration by far is the quantity and quality of the surface you provide for the nitrifying bacteria to inhabit. The material that fills the filter box and on which the bacteria grow is referred to as the biological filter medium. I strongly recommend that you use 1-inch stone, 1- to 2-inch lava rock or one of the new high flow-rate plastic materials for the biological filter medium. Avoid using sand or pea gravel, which can be a maintenance nightmare. Of the three types of material, I find plastic medium to be the best. I use it exclusively now in all the filters I design and build because 1) it offers a high surface area for bacteria with a small volume of material, 2) it does not clog readily but is easily cleaned when it does, 3) it is light and easy to work with and 4) it has good water flow characteristics (lots of void space in the medium). The one drawback of plastic medium is that it is rather expensive.

Second place goes to lava rock. Lava rock is preferable to ordinary stone because it has more surface area per unit volume than ordinary stone, and it is far lighter. Large lava rock medium (over 1 inch) does not seem to clog as quickly as ordinary stone. Third place goes to standard stone medium. In any case, the designs presented here will work with any of the three materials I recommended.

Let's examine in more detail what occurs around the filter medium. There is a thin bacterial film that coats the surface of each piece of medium. In order for the nitrifying bacteria to act on the ammonia in a given volume of water, the water flow must come into contact with the bacterial film. The water must also be in contact with the bacterial film for a minimal period of time for complete conversion of the ammonia to take place. Thus, a very important design consideration is the contact time of the pond water with the filter medium.

Many variables can affect the contact time required for maximizing the effectiveness of nitrification in a biological filter: water temperature, pond volume, size and surface characteristics of the medium and void space in the medium, to name a few. A conservative method for estimating the volume of medium required to ensure a minimum contact time that yields 100 percent ammonia conversion for your pond is to divide the pond volume in gallons by 125. The result is the required medium volume given in cubic feet. In almost all cases, the filter will have extra capacity. In the case of 5000 gallons of water, for example, the filter box will need to hold about 40 cubic feet of medium.

Next you need to determine the flow rate of the pond water through the filter. This is a very important determinant of the average daily background ammonia levels in your pond (see the sidebar to the right). The fact is, the actual ammonia level in a fully recirculating pond is never zero — even if the filter design is so efficient in design as to ensure 100 percent ammonia removal. There is always some amount of ammonia in the pond water because the fish are continuously adding ammonia to the water, but the filter can only remove ammonia from that small portion of the pond water that is moving through it at any given time. So even as one portion of the pond water is being cleansed of ammonia, another part is being polluted. From this explanation it should be obvious that the greater the number of pond volumes moved through the filter each day, the lower the average ammonia level will be. (If the filter were not 100 percent effective in removing ammonia, the average level would be even higher!)

Total Ammonia Levels (ppm) At Various pH And Temperature Values
(degrees Fahrenheit)
pH Values

For example, let's assume that the 5000-gallon pond is stocked to the point where the daily production rate of ammonia is approximately 9500 milligrams. This means that about 0.5 ppm of ammonia is added to the water each day (9500 milligrams divided by 18,925 liters or 5000 gallons). If only 5000 gallons per day is pumped through the filter — that is, one pond volume turnover — the average ammonia levels will be 50 percent of 0.5 ppm: 0.25 ppm. You will always measure about 0.25 ppm in the pond water no matter how big the filter! Now, if the flow rate is increased to five pond turnovers per day, the average ammonia level will drop to 10 percent of 0.5: 0.05 ppm, at which point you will not obtain a measurable reading with an ammonia test kit. In other words, pond volume, filter volume and filter flow rate are intimately connected. In a properly designed system, the average ammonia level, while never zero, is simply too low to be measured with hobbyist test kits.

In order to ensure a reasonably low average ammonia level, you should design your filter to process at least 12 pond turnovers per day. Thus, one-half of the pond's volume will be pumped through the biological filter every hour. One full pond volume per hour (24 turnovers per day) would be even better, but the costs for the pump and electricity become prohibitive with very large ponds. To determine a rate of one-half pond turnover per hour, just divide the pond volume in gallons by 2. This will give you the flow rate entering (and exiting) the filter in gallons per hour. For a 5000-gallon pond with a flow rate of one-half pond volume per hour, you would want to pump 2500 gallons per hour, whereas for one full pond turnover per hour, you would need to pump 5000 gallons per hour.

With the volume of filter medium and the filter flow rate (pond turnover rate) decided, the third step is determine the surface area (the length times the width) of the filter box that holds the biological medium. Although there are an infinite number of combinations of surface area times depth that will yield a given volume, only certain combinations offer maximum filtration efficiency. In particular, you want to match the filter surface area to the flow rate entering the filter so that there is 1 square foot of surface area for every 1 to 2 gallons per minute of flow. This relationship of flow rate to unit of surface area — dividing the gallons per minute of flow by the filter surface area — is called the hydraulic load on the filter bed. Basically, by designing your filter for this range of hydraulic loads, you will ensure that the water moves slowly enough through the filter so that the ammonia and nitrite are completely removed, but not so slowly that water that has already had these substances removed is still sitting in the filter box taking up space.

Pond Turnover Rates
and Average Ammonia

You should calculate the turnover rate of the pond through the filter. This is simply the volume of water your pump moves per day (gallons per day) divided by your pond's volume in gallons. For example, if a pump moves 1000 gallons per hour, it would be pumping 24,000 gallons per day. With a pond volume of 5000 gallons, the turnover rate would be 4.8 pond volumes per day.

The turnover rate (4.8 is this case) determines the average ammonia concentration (the ammonia remaining in the pond after filtration). The average ammonia level will vary according to the number of turnovers per day.


The final step is to determine the depth of the filter medium in your filter box. Depth is simply the volume of filter medium divided by the filter surface area. Because we have systemized our filter design relating pond volume, filter volume, filter flow rate, surface area and hydraulic load, the depth of the filter medium will always be fixed at one of two values. If you have decided to circulate one-half the pond volume per hour through the filter, then the depth of the medium will always be 18 inches. If you decide on one complete pond volume turnover per hour, then the medium will always be 12 inches deep. Either of these depths will ensure good flow characteristics through the filter bed without making the filter too prone to clogging and channeling.

There are several other design considerations worth noting. Always build the biological filter to operate outside of the pond. Pumping water from the pond, through the filter and back into the pond allows for more efficient designs and fewer problems. Pond filters that are built directly into a pond are a continual source of operating difficulties and endless maintenance problems.

Some pondkeepers make their filters larger than my design recommendations would require. They do this to compensate for the partial clogging and channeling that occurs in the filter bed over time. At best, however, this strategy merely lengthens the time between clogging — clogging will still occur. With the possible exception of some designs of plastic medium, most types of filter medium will inevitably clog to varying degrees. Unless you have an efficient and effective way to clean the filter bed, it will eventually clog to the point where the filter is no longer working effectively. The result will be a sudden, catastrophic rise in ammonia in the pond.

If you choose to use plastic medium, removing accumulated material in the filter bed is easy. Simple back flushing with a hose is sufficient to clean the bed. When using stones or lava rock, a straightforward solution for cleaning the medium every few weeks is to include an air-blower system when building the filter. Essentially, this is just a network of perforated pipes that rests on the filter grid below the medium, with an inlet blower connection. When the blower is turned on, air is forced to bubble up through the filter bed, carrying away trapped particulate matter. The waste water carrying the freed solids is then directed to overflow into the garden, not into the pond! This material is an excellent garden fertilizer, but will only add to the biological load in the pond if returned there. A drain at the bottom of the filter will aid in removing accumulated particulate matter at the bottom of the filter box.

To help slow down the rate of clogging, the use of a settling basin, explained in the first half of this article, is highly recommended. By placing the basin so that pond water flows through it before entering the biological filter, you can reduce — but not eliminate — the rate of clogging.

Biological filtration consumes about 5 milligrams of oxygen for every milligram of ammonia converted to nitrate. Other bacteria that live on the filter medium consume another 3 to 5 milligrams of oxygen per milligram of ammonia converted. As a result, a biological filter serving a heavily loaded pond with high ammonia production will consume a lot of oxygen. Therefore, your pond filter system should include a means for aerating the water as it exits the filter. A common approach is to design the filter outlet so that it spills into an aerating waterfall or stream. The effect of water flowing over a series of rock or gravel cascades is to remove carbon dioxide from the water while adding much needed oxygen. The more "steps" in the flow path, the better. Water that falls from a great height directly into the pond may look dramatic, but the aerating effect is only marginal.

An alternative biological filter design — one that incorporates aeration internally — is the trickle filter. In this design, the medium is not submerged under the water. Instead, the incoming pond water is dispersed over the top of the medium via a network of perforated pipes or a spray bar. The water then trickles down through the filter medium and exits at the bottom. The biological medium in the trickle filter is always covered by a thin coating of water running over the surface, but the spaces between the medium material are not flooded. Aeration occurs in the filter bed as the water cascades down the medium surface, bringing air into the bed itself. A trickle filter requires exactly the same design parameters as a submerged filter for a pond of a given size and biological load. Contrary to popular opinion, a trickle filter is not intrinsically more effective than a submerged filter. The single advantage it offers over the submerged biological filter is superior aeration within the filter bed itself.

Operating Considerations
It takes about five to six weeks in warm weather (around 70 degrees Fahrenheit) for a newly established biological filter to reach full operating capacity. In cooler weather — below the mid 60's — it can take 10 weeks or more. Trickle pond filters will take longer than submerged filters to become fully operational in cold weather because evaporation on the medium surface results in further cooling. In any case, it is important to monitor ammonia and nitrite levels carefully during this period. Water changes might be required to keep levels within safe limits.

Once the filter is operating, it is essential that it be kept running continuously. While it is acceptable to shut down a submerged biological filter for several hours for cleaning, modification or repair, pond ammonia levels will certainly rise if it is left off much longer. A trickle filter, in contrast, is always in danger of drying out if the water flow is cut off (this is not a problem with submerged filters if water is kept in the filter chamber). Never let the biological medium dry out during maintenance.

In the fall, as temperatures become lower, the nitrifying capacity of a biological filter drops as well. The nitrifying capacity of a trickle filter is reduced at an even faster rate than a submerged filter. Fortunately, the ammonia production rates of koi and goldfish are also reduced as temperatures fall, and ammonia toxicity is therefore significantly less. As long as the flow of water through the filter is maintained, things should remain in balance. However, those of you who — like me — live in areas with very cold winters have no choice but to turn off the filter as freezing temperatures approach. If the pond is not overloaded, serious ammonia problems will not occur. Shutting down the filter will not harm the fish because they do not eat while over-wintering, and thus produce relatively little ammonia. Unfortunately, this does mean that you must restart the filter from scratch in the spring.

In the spring, there are numerous changes in the pond that can present problems, the result of the fish becoming active long before the biological filter reestablishes itself. I strongly recommend monitoring ammonia and nitrite levels closely during the spring. You should never allow the pH of the pond water to drop below 6.5. Besides the fact that most pond fish — particularly koi and goldfish — do not thrive in highly acidic water, nitrification is also inhibited as the pH drops. If you have soft, acid water in your area, you should consider adding a mesh bag of crushed oyster shells to the pond filter. This source of calcium carbonate will increase the pH and help maintain it there.

Be aware of the nonsense factor in pondkeeping. For example, in recent years, some silly pieces of advice have appeared in hobbyist newsletters, including the assertion that nitrifying bacteria need sunlight to thrive. This is ridiculous. In fact, sunlight actually inhibits nitrifying bacteria.

Finally, keep in mind that an active and healthy pond is a dynamic system. Success in raising koi and goldfish can ultimately lead to disaster if you do not adjust for the changing biological load. Fish grow, and, over time, what was once a low fish load, can become a high fish load even without the addition of more fish to the pond. With koi, for example, every time they double in length their mass increases by more than eight times! If you had five 4-inch koi in your pond last year and this year they have each grown to 8 inches, the fish load is not twice as much, it is eight times greater! Goldfish mass also increases much faster than body length, with wide variations among varieties. In mechanically and biologically filtered ponds, I suggest never allowing the load to surpass 2.2 pounds of fish per 265 gallons of water. This is approximately two 13-inch koi. Monitor your pond regularly. Measure pH, ammonia, nitrite and temperature.

There are innumerable variations on pond filtration. The goal of this two-part article has been to make you aware of the rudimentary concepts in filter design. It is important to keep in mind, however, that no filter system — no matter how sophisticated — can return pond water to pristine conditions. Filters merely slow down the decline in water quality. The best single thing you can do to maintain the kind of water quality in which your fish will remain active and healthy is to keep the fish load low. The lower the load the better. Your pondkeeping goal should be to see your fish thrive and live out a full, normal life (up to 20 years for goldfish and well over that for koi), not to see how many fish you can cram into a pond for short periods of time.

Although pond filtration can allow you to maintain a fish pond with loads considerably greater than nature would otherwise allow, there are limits. As long as you stay within those limits, a pond filter can help you keep your fish healthy, and in doing so make your pondkeeping all the more enjoyable.

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