Pond Water Quality is Essential
There's really no magic or mystery to pond water quality.
Stephen M. Meyer
As every beginning fish and pond hobbyist will eventually learn, the key to fishkeeping success is maintaining good water quality. However, this is often easier said than done.
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Suspended algae causes murky pond water. Photo by Thinkstock
The typical ornamental pond operates as a 100-percent recirculating system (just like the traditional home aquarium), with the same water remaining in the pond for weeks or months at a time, even when it rains frequently. In this situation, water quality is always a problem because as time passes, various physical, chemical and biological processes working in and around the pond alter the initial "pristine" characteristics of the water.
A variety of natural pollutants accumulates, especially when there is a good-size population of fish in the pond. Increasing amounts of solid matter float on the surface, and sooner or later come to rest on the pond bottom. Dissolved substances of all types build up to concentrations that affect the health of plants and fish. Too often, the inattentive fishkeeper is first alerted to these changes only after his, or her, favorite aquatic pet is found floating belly up, or that rare water lily from Brazil has dissolved into oblivion.
Although no filter can maintain the superb conditions of an open aquatic system, such as a stream-fed pond, an appropriately designed pond filtration system can significantly slow the decline in water quality. Here, the term "appropriately designed" means a filter that fits the needs of the individual pond and pondkeeper — not a high-tech wonder that requires its own engineering crew for maintenance. Most importantly, it does not transform the would-be pondkeeper into a permanently enslaved filter keeper. With these goals in mind, this article will examine the most effective options for no-hassle pond filtration.
To accent my notion of "appropriateness," I have built this discussion around two basic types of ponds: garden ponds and fish display ponds. A garden pond — often called a water garden — is an attempt to mimic nature in certain ways. Most importantly, the primary form of pond life is plants, not fish. Indeed, there may be no fish at all. Frequently, a garden pond is designed as a landscape feature that will attract wildlife as a result of the pond layout, the near-pond topography and plantings planned to maximize variety of shelter and food.
A typical garden pond might be kidney-bean-shaped — 10 feet long, 6 feet wide and 2 feet deep — and hold about 600 gallons. Smaller and larger garden ponds are common. Plantings would include several water lilies, a variety of submerged plants, such as Anacharis, Cabomba and Myriophyllum, floating plants, such as water lettuce or water hyacinth, and assorted emergent plants — iris and horsetail — rising out of the water. Also inhabiting the pond might be a pair of goldfish.
A fish display pond, as the name implies, is designed and operated 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. Fish loads are quite high — 10 to 100 times what you would ever find in nature. A typical koi pond might hold 2500 gallons and host 40, or more, adult koi. Filtration for such ponds is not an option — it is the fundamental life-support system for the fish.
Problem #1: Solids
In the typical ornamental pond exposed to the elements year round, solids can mean anything from microscopic particles of dust and algae to large objects, such as leaves, Styrofoam cups, tree branches and automobile tires. Some of this material is transported by wind and rain. Most of the vast colonies of microscopic plants and animals that eventually inhabit your pond arrive this way. So do yard trash and debris. Some is deposited by animals that visit the pond, some by your neighbors. But a very substantial amount is produced by the biological and ecological processes that make your pond a living system. Algae reproduce at fantastic rates, plants in and around the pond shed leaves, and fish excrete solid wastes.
From the aquarist's perspective, all this stuff would have to go — the water should be sparkling clear, like bottled water. But a pond is not just a large aquarium that is placed outdoors. It runs according to a different set of ecological rules. The extent to which you should be concerned about the deposition and accumulation of solids in your pond depends greatly upon the purpose and goals you have set for your pond. It goes without saying that trash and debris, such as plastic bottles, newspaper and dead birds, should be removed promptly from pond waters. But, from then on the guidelines are largely subjective.
In terms of purpose, a fish display pond is the closest thing to an indoor aquarium. Koi ponds, in particular, are often designed in this manner. Beyond maintaining good water quality for the fish, the particular ecological and habitat value of the pond is not relevant. Because these ponds are, by definition, heavily stocked (i.e., overloaded with fish), efficient removal of accumulating fish and food solid wastes is a top priority. Allowing this material to remain in the pond can significantly increase ammonia concentrations, reduce dissolved oxygen levels and increase the risks of disease.
A clear view of the fish is also a top priority in fish display ponds, and therefore it is vital that even fine suspended particles — the stuff that makes the water look murky — be removed effectively. As one pond owner said to me: "...I paid more than $12,000 for these koi and I sure as hell am going to be able to see them whenever I like."
At the other extreme is the garden pond, in which solids, large and small, are an integral part of the garden pond's ecology. Leaves and branches are allowed to float on the surface and sink to the bottom. Decay debris (detritus) accumulates on the pond floor. The water column is transparent, but tinted with a strong greenish-brown cast — a sign of thriving bacterial and planktonic colonies. The water is not dirty, it is alive. Water striders, dragonflies, snails, snakes, frogs, salamanders and birds of many kinds all find a reason to take up residence. An overdeveloped sense of cleanliness in this instance would defeat the very purpose of the pond.
Removing Solids from Pond Waters
In the home aquarium, solids are removed by some form of mechanical filtration. Filter floss, foam sponges, fiber cartridges, undergravel filter beds and, on occasion, diatomaceous earth, are among the most commonly used media.
Today, pondkeepers have an equally large array of mechanical filtration options from which to choose. Commercial products include huge sponge filters, floss, foam pads, high-rate (swimming pool) sand filters, fiber cartridges, gravel beds, screens, brushes, settling basins and so on. While, in theory, all of these will work for a short period of time, most are not suitable for long-term, low-maintenance operation outdoors.
The wise pondkeeper will approach the task of solids removal using three basic methods: 1) manual removal, 2) coarse screening and 3) settling. Together, these three techniques are very effective in keeping solid matter at levels appropriate for outdoor ponds while having low maintenance demands.
It might seem silly to mention manual removal as a mechanical filtering technique, but it is both highly effective and essential. Manual removal of leaves, twigs, branches, yard debris and trash from your pond by hand is the only mechanical filtration technique that literally ends contact between this matter and your pond water. All other methods merely trap the debris while holding it submerged. Wind-blown debris may carry with it toxic contaminants (e.g., leaves with persistent insecticides, or grease-laden paper towels) that could leach into pond water. Physical removal, rather than simple trapping, is prudent.
Then too, large solid matter that remains in contact with the water is eventually broken down into smaller suspended fragments by physical, chemical and biological action. These smaller suspended and settled solids are much harder to remove from pond water and may cause the water to take on a murky appearance.
A good skimming net on a telescoping pole and a few minutes each day is all you need. Stop by any swimming pool store. They will carry a range of skimming and leaf-bagging nets that can be fitted to an adjustable aluminum pole. For garden ponds this may be the only form of solids removal necessary or desirable. For fish display ponds, however, manual removal of solids is just the first step.
Some type of coarse screening should overlap manual removal in trapping larger solids, such as leaf fragments, pine needles, small twigs and moderately sized insects. Where manual removal is intermittent, coarse screening is ever vigilant. If you are operating a circulating pump (which you should) then you already have a modicum of coarse screening. Most external (non-submersible) pumps incorporate a strainer basket that uses a coarse mesh to prevent solids from entering the pump chamber and jamming the motor. Submersible pumps usually have a contoured screen that covers the pump inlet.
While these screens may be adequate if cleaned daily, I find them too small to suit my tastes. What if you come down with the flu, or want to go away for a few days? I prefer to limit my cleaning chores to twice a month — at my convenience. If you use an external pump for water circulation, you might consider constructing a simple "mega-strainer basket."
If you use a submersible filter, there are two possibilities to consider. Placing the pump in the bottom of a 5-gallon pail and filling the pail with stones is a quick, simple and effective option. There is one major drawback: cleaning requires a strong back to lift the pail full of stones out of the pond for rinsing and flushing. Alternatively, a plastic meshed clothes hamper makes an excellent screening chamber for submersible pumps. Surround the hamper with cloth window screening, submerge the hamper in the pond, put the pump inside the hamper and cover the open end of the hamper with more screen. Admittedly it is not attractive, but it does the job.
Still another option is to buy one of the commercial submersible filter tubes that is available (e.g., Danner Manufacturing and Liliponds both have units). Basically, this is a rigid grill tube surrounded by a foam pad. Remove the foam pad and replace it with window screening.
The third method of solids removal, settling, is provided by mother nature. Solids denser than water leave suspension and drift to the sides and bottom of the pond under the forces of gravity and friction. You may have noticed that pond murkiness (turbidity) frequently increases in the aftermath of a heavy rainstorm as debris is washed into the water and previously settled materials are stirred up. After things quiet down, subsequent settling activity leads to substantial clearing in a few days.
In-pond settling can be very effective in garden ponds and lightly stocked fishponds where turbulence from the recirculating water and the fish is low. As wetland studies have shown, planting dense rows of emergent plants, such as iris or cattail, near the water inlet and outlet — acting as a vegetative screen — reduces turbulence and can significantly enhance settling of even very fine particles. Levels of suspended matter drop noticeably after a few days and the water column remains remarkably clear.
Contrary to popular wisdom, the material that settles on the garden pond floor does not have any deleterious effects on the pond, assuming that some water recirculation is provided for basic aeration (which should be the case in all garden ponds). It does not harbor "bad bacteria" or poison the water (who makes up this stuff?). In fact, it can contribute directly to the pond's habitat value by providing a home for many types of invertebrates, rooting medium for plants and hibernation shelter for reptiles, amphibians and fish.
Yes, that's right, fish. My pond comets (goldfish) burrow down into the stuff as the 18-inch-thick ice sheet forms over the ponds. So do the frogs and dragonfly larvae that live in our ponds. Not one has died, or even become ill, in more than a decade. And my bare-root Japanese irises bloom like crazy after a season with this material coating their roots.
Of course, those afflicted with a cleaning fetish can always use a pool vacuum to remove the settled material. But that is somewhat like vacuuming the soil from a vegetable garden.
In-pond settling in fish display ponds, however, is not effective. These ponds are always turbid because the extreme and continuous levels of fish activity never allow settling to occur. All except the heaviest particulates remain in suspension. This is why fish display ponds and overstocked ponds require much more substantial mechanical filtering to attain the same clarity as garden ponds. Moreover, the quantity of solids is very large in fish display ponds, and microbial action will remove oxygen from the water while increasing ammonia levels — factors that reduce water quality.
Taking advantage of settling in these ponds is possible if a special settling basin is constructed outside the pond. The settling basin is a transitory waterway in the recirculation system that is big enough to allow solids to settle out before the water returns to the pond.
How big should the external settling basin be? All else being equal, the time required for a given particle to settle out depends on its density, shape and size. Heavy, small and round particles settle out faster than light, larger and flat particles. In general, the larger the settling basin volume in relation to the pond volume, the greater the amount of solids that will settle out in the basin.
What little research has been published suggests that settling times of 15 minutes or more are required to remove heavier matter. Thus, we want the pond water to spend at least 15 minutes moving through the settling basin. Because my rule of thumb for recirculation flow rates is that an entire pond volume should be recirculated at least once every two hours, this suggests that the settling basin volume should be roughly one-eighth the total pond volume. For instance, a 1000-gallon pond should have a settling basin that holds about 125 gallons. Very heavily stocked fish display ponds should have 1-hour turnover rates, implying a basin that is one-quarter the volume of the pond.
The basin should be about 2 feet deep along the side where the water enters, incorporate a baffle that produces laminar flow, and have a floor that slopes upwards toward the end where the water exits. A drain valve should be placed at the deep end for flushing accumulated material.
The effectiveness of a settling basin can be improved further by placing intermittent blockages in the water flow, as shown in the diagram. Brushes are increasingly popular. My preference is to use emergent plants — such as irises — aligned in dense colonies. Water hyacinth also work well in this capacity. Not only are plants very effective in this role, but they also provide other filtration benefits (discussed below).
Keep in mind that the solids removed by coarse screening and settling remain in contact with the pond water and are still "available" for bacterial decomposition, which can affect water quality (positively or negatively). Thus, until this material is physically removed from the system, removing solids from suspension is almost entirely cosmetic. In a garden pond this is fine — even desirable. In a fish display pond it can be very unhealthy for the animals. Consequently, the settling basin for a fish display pond should be drained daily.
The last point I would mention involves a particularly vexing suspended solids problem for pondkeepers: planktonic algae. This is the cause of pea-soup-green ponds and, believe it or not, planktonic algae (alive and dead) often account for the bulk of solids in ornamental ponds. Even in garden ponds, where a healthy crop of algae is desirable, too much of a good thing can inhibit the growth of water lilies and submerged plants and skew the pond's biology in unintended directions. In fish display ponds, of course, the fish become totally invisible.
The cure for this solids problem is simple and 100-percent effective: install an ultraviolet sterilizer. The unit should be sized to the flow-through of your recirculating system in order to provide at least 15,000 microwatt-sec of exposure. Pond ultraviolet sterilizers should always include quartz sleeves to protect the bulbs.
Things to Avoid
Unless you have an inordinate fondness for filter maintenance, avoid sand filters, fiber cartridge filters and diatomaceous earth filters. These mechanical filtering devices are fine for chlorine-treated swimming pools but they make no sense for ponds. First, no pond needs the degree of particulate removal these systems provide. Second, they are so effective at removing fine particles that they demand daily cleaning. Some even require backflushing several times a day. Remember, you are a pondkeeper, not a filter keeper.
Problem #2: Nitrogenous Wastes
Perhaps the most serious problem facing the fishkeeper is controlling the biologically produced nitrogenous wastes — in particular, ammonia. Excessive concentrations of ammonia can kill fish and many other forms of aquatic life. At lower, non-lethal concentrations, ammonia can cause chronic stress that reduces a fish's ability to fight disease, cuts food consumption and growth, inhibits reproduction and ultimately contributes to premature death. Thus, it is very easy to understand why aquarists and pondkeepers are so preoccupied with this problem.
However, the significance of nitrogenous wastes to pondkeeping is more subjective than in the case of the traditional home aquarium. On the one hand, in the basic garden pond they are irrelevant. You will never get meaningful concentrations of ammonia in an 800-gallon pond stocked with plants and a pair, or two, of goldfish. The plants will soak it up as readily as it is produced. (Yes, that's right, plants take up ammonia much more readily than nitrate. In fact, they must break nitrate down to use it.)
On the other hand, failure to control nitrogenous wastes in a fish display pond, as in a heavily stocked fish tank, means disaster. This is because the amount of nitrogenous waste produced is many times greater than the pond's natural capacity to absorb it.
You cannot have a biologically active aquatic environment — aquarium or pond — without having ammonia constantly being added into the water. One of the basic bacterial activities in any aquatic environment is the breaking down of organic matter through ammonification. Formerly living materials become, in part, ammonia. Fish continuously excrete ammonia through their gills, as well as through dilute urine. Other pond animals also release ammonia into the water.
The issue is the balance between what is produced and what is removed. In natural ponds this problem is all handled by a simple ecological balancing act. There are simply more consumers of ammonia than there are producers.
Ammonia is taken up by plants (including algae) as their primary food source. Competing with plants are Nitrosomonas spp. bacteria, which in the course of using ammonia convert it to a different nitrogen compound: nitrite. This compound also turns out to be quite deadly to fish, often showing serious side effects in concentrations as low as 0.1 parts per million. However, before it can reach harmful levels, another bacteria — Nitrobacter spp. — takes up the nitrite and transforms it into nitrate. This nitrogen compound is not harmful to fish or other aquatic life in concentrations that tend to accumulate in natural or ornamental ponds (even up to several hundred parts per million).
Consequently, in a garden pond, you simply don't have to worry about this problem. Vegetative filtration, supplemented by nitrifying bacteria in the pond, takes up all the available ammonia. In fact, in garden ponds lacking any fish there may be too little of this plant food available, requiring the pondkeeper to "fertilize" the pond water.
The situation in the fish display pond is much more desperate and fragile. The average goldfish or koi produces about 25 milligrams of ammonia per day per 100 grams of body mass. The proportion for other fish species is not too different. Thus, if you have 20 average-size (700-gram) koi in your 1000-gallon pond, you have at least 3500 milligrams of ammonia entering the pond water each day. That is almost 1.0 part per million per day. After a few days this problem will take care of itself because the fish will be dead.
When the fish load exceeds the natural capacity of the pond, the only practical solution is to install a supplementary biological filter. (A safe estimate of the natural capacity of an ornamental pond, well planted, is one average-size goldfish per 10 square feet of surface area.) A supplementary biological filter is little more than a waterproof box outside the pond that holds the media on which nitrifying bacteria can grow. Some people build them out of concrete, others use trashcans.
One solution is a basic down-flow, submerged biological filter. (It is called a "submerged" filter because the filter media are always underwater.) It is a simple box in which water is splashed across the top and flows down through the filter media before exiting to the pond.
Today there are many commercial biological filters on the market. Some are first rate and will get you up and running quickly. Others are just plain rip-offs. Let me suggest some design principles that should be common to all filters.
The first consideration is the quantity and quality of the media surface you provide for the nitrifying bacteria to inhabit. I prefer light plastic bioballs. One-inch pieces of lava rock or stone also work well. Sculpted foam blocks are popular now. The old standby is ½-inch gravel.
I prefer plastic media like bioballs because 1) they offer a high surface area for bacteria with a small volume of material, 2) they do not plug readily, but are easily cleaned if they do, 3) they are light and easy to work with and 4) they have good water flow characteristics (i.e., lots of space in the media). The one drawback of plastic media is that they are very expensive. However, because they last forever, the cost spread over 10 years or more is very low. (Don't believe the argument that bacteria have trouble sticking to plastic media. You can't judge the bacterial quality of a surface by its smoothness.)
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 media (more than 1 inch) do not seem to clog as fast as ordinary stone. Third place obviously goes to standard stone media.
I strongly advise staying away from some of the self-proclaimed "super-media" composed of fiber batts, pads and so on. They will not work for long without daily cleaning. Bacteria, algae and detritus will quickly seal their open spaces. If the media looks like it would be good for mechanical filtration — it will be. Avoid it completely for biological filtration.
If you choose any of the filter media I suggested, the biological filter should hold about 10 percent of the volume of the pond itself. Therefore, a biofilter for a 1000-gallon pond should hold about 100 gallons of media. Using this rule of thumb will give you plenty of filter capacity to carry you through most circumstances.
The next design variable to consider is the flow rate of the pond water through the filter (i.e., the recirculation rate). This is a very important determinant of the average daily background (ambient) ammonia levels in your pond. The fact is that the actual ammonia level in a fully recirculating pond that requires supplementary biological filtration is never zero — even if the filter design ensures 100-percent ammonia removal effectiveness. There is always some amount of ammonia in the pond water because the fish are continuously adding ammonia to the water. 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 will be the ambient ammonia level. (If the filter were not 100 percent effective in removing ammonia, the ambient level would be even higher!)
In order to ensure a reasonably low ambient ammonia level, you should plan for at least 12 pond volume turnovers per day — one complete pond volume pumped through the biological filter every two hours. One full pond volume per hour (24 turnovers per day) would be even better — and is necessary in heavily stocked ponds — but pump and pumping costs become prohibitive with very large ponds. A pump for a 1000-gallon pond, therefore, would have to move between 500 and 1000 gallons per hour. Please note, this is the actual flow through the system after considering pipe resistance, uphill pumping and so on.
Because a supplementary biological filter is a big consumer of oxygen, you must be sure to include substantial aeration capacity in your filter system. It does not matter whether you set up a large cascading waterfall or install air blowers and venturi jets. But you must provide considerable aeration.
Vegetative filters can play an important role in removing nitrogenous wastes from fish display ponds as well. First, as already noted, plants remove ammonia directly from the water. Second, trimming or harvesting the plants removes nitrogen directly from the pond system. New growth, stimulated by the trimming, removes still more ammonia.
Wetland studies have also shown that the submerged sections of plants and bare plant roots are a fabulous medium for nitrifying bacteria. By hosting these nitrifiers, the vegetative filter does double duty in removing nitrogen wastes — and no artificial media to clean.
Things to Avoid
Never build the filter inside the pond. These "in-pond" filters are a source of endless operation and maintenance problems.
Avoid narrow and deep filter designs. The larger the top surface of the filter and the shallower the depth, the better the filtration and the fewer the maintenance problems. Specifically, deep biological filters act as excellent mechanical filters/settling basins. The 100-gallon filter noted above would represent a little more than 13 cubic feet. A box that was 6 feet long, 2½ feet wide and 1 foot deep would be perfect.
Again, avoid very fine media. Unless, of course, you like cleaning filters.
Do not use zeolites or other ion exchange media. While they may be fine for indoor aquariums, they will not work for long in an outdoor environment. After a week or so the media will be coated with bacterial slime, algae and detritus that will effectively prevent adsorption of ammonium ions. At that point the media become essentially inert and therefore little more than small and very expensive gravel stones.
Never shut off the recirculating system at night. It should run 24 hours a day.
Problem #3: Dissolved Organic Compounds
Dissolved organic compounds (DOCs) are carbon-based substances (excluding carbon dioxide and carbonates) that are the metabolic byproducts of pond life. DOCs include carbohydrates, proteins, amino acids, fats and phenolic compounds. Concentrations of DOCs may be under 1 part per million in clean rivers and lakes, but well over 100 parts per million in polluted waterways and certain stagnant ponds.
DOC levels in garden ponds tend to be fairly low and are composed largely of plant-derived substances. The worst effect on the pond is the addition of a yellowish-brown tint to the water. The dominance of plant-derived DOCs means that cellulose-decomposing and starch-consuming bacteria will comprise the majority of the water's heterotrophic bacterial population, which is what you would want in a water garden. Therefore, DOCs are irrelevant to the pondkeeper tending a garden pond.
In contrast, DOCs in fish display ponds are overwhelmingly fish derived. Controlled studies show that these forms of DOCs suppress fish growth and impair immunity functions. Moreover, these DOCs support rich populations of animal protein-eating bacteria, a number of which are pathogenic to fish. Thus, the risks of infection and disease increase in fish-derived DOC-rich waters.
The implications of all this are clear. Fish display ponds must have some mechanism for controlling the relative concentrations of DOCs. In aquariums this is easily and efficiently handled by the addition of granular activated carbon to the filter box. Unfortunately, this is not a practical approach in ponds. The quantity of carbon required for a 1000-gallon pond is prohibitively expensive — about 10 pounds a month. But even for those with more dollars than sense, carbon is not particularly effective because it quickly becomes fouled with bacteria, algae and detritus, rendering it useless.
Frequent partial water changes will, of course, keep DOC levels down. But this too can get very expensive with large ponds, and many parts of the country experience water shortages that limit such activity. Moreover, if your fishpond is heavily stocked, nothing short of a 50-percent water change every few days would make much of a difference.
A practical option for pondkeepers with heavily stocked fish ponds is a foam fractionator. This is a simple device that uses a column of air 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 and around the edges is caused by bubbles coated with DOCs.
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 an air bubble 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 available commercially, but they are also quite simple to build using ordinary PVC pipe. (Complete construction details are given in Aquarium Fish Intl., April 1992.) As shown in Figure 4, water enters the foam fractionator through a standard spa jet (available at pool and spa retail stores). 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 2 hours.
The venturi in the jet causes a pressure drop as water moves through the nozzle, and air will enter the stream at the pressure drop. The air-water mixture moves up the fractionator column and adsorption occurs. This DOC-enriched foam collects at the top of the column and ultimately builds up to point where it spills out through the exit port. The water then moves back down the column in a counter-current flow and is stripped of additional DOCs before finally exiting the column.
Depending on the conditions in your pond, it may take a few hours or a few days before a foam fractionator begins to produce results. This simple device can significantly improve water quality and fish health in heavily stocked fishponds. It is not, however, a substitute for proper stocking levels. The fish in a heavily stocked display pond that uses a foam fractionator will never be as healthy as those in a lightly stocked garden pond lacking supplementary filtration.
It should be obvious that answer to the question "Do I need a filter for my pond?" really has only one answer: it depends. The basic garden pond or water garden requires nothing more than occasional manual removal of debris and regular partial water changes. An "awesome" fish display pond needs several forms of fairly demanding filtration technology, as well as larger partial water changes.
Your pond probably falls somewhere in between these two extremes. The points raised in this article should help you to decide whether additional filtration is prudent, and if it is, what type makes sense for you.