To filter or not to filter, that is the question.
Stephen M. Meyer
Q. I'm trying to decide if the pond I'm planning will need a filtration system. I haven't even determined the exact size of the pond, but I think it may be close to 2500 gallons. I assume that the larger the pond, the less likely it is that I'll need a filter. If I do need a filter, is it the pond volume or the fish bioload that determines its size? More generally, what are the basics of pond filtration and what do I need to take into consideration? Thanks.
A. Pond filtration has become increasingly important over the years, for several reasons. First, pondkeepers often maintain too many fish in their ponds, and as a result, the natural biological processes are unable to cope with the wastes produced. Second, the increasing cost and scarcity of water, which is a result of increased demand coupled with lower than normal snow and rain in many parts of the country, has made recirculation of pond water through a filter a necessity. The use of continuous-feed water systems in which the pond water is replaced every day is no longer practical or cost effective. Third, many pondkeepers want clear, clean pond water for purely aesthetic reasons.
The basics of pond filtration are similar to those for aquarium filtration. Mechanical filtration is used to remove suspended particles from the water, and biological filtration is used to remove nitrogenous wastes that are dissolved in the water. As your question correctly suggests, choosing filters that are the appropriate size for the pond is crucial if they're to be effective.
Notice that I said filters, meaning more than one Owners of very small garden ponds containing no more than 250 gallons of water can limit filtration to a submersible pump in a 5-gallon pail filled with 10-millimeter gravel and placed on the bottom of the pond. This will work quite well as both a mechanical and biological filter, although it may have to be removed for daily cleaning. Larger ponds should have separate filters for each function, with water passing through the mechanical filter before it reaches the biological filter. Let's consider this in greater detail.
In addition to solid wastes produced by the fish, many things fall into the pond over the course of a season, including bugs, leaves, paper, tree branches and other things. Some items will settle to the bottom of the pond, while others will float at the surface. Natural decay will eventually break down these items, resulting in a layer of material on the pond bottom. This material can be easily resuspended in the water by the slightest disturbance, clouding the water.
Because there is so much suspended material, much of it of different sizes, filters for ponds should be designed for coarse filtration. If the filter is capable of removing very small particles, it will clog much more rapidly and therefore require significantly more maintenance.
There are numerous filter designs available. One inexpensive but very effective mechanical pond filter is the Supreme Poolmaster from Danner Manufacturing (160 Oval Dr., Central Islip, NY 11722). It consists of a foam sleeve around a hollow cylinder and a submersible pump to draw water through the foam-covered cylinder. The filter can handle ponds up to 500 gallons in size and is virtually silent. For larger ponds, several of these pumps can be ganged in parallel. The one drawback is that the foam sleeve is such a good mechanical trap, despite its small size, that it needs to be cleaned every two days or so.
Another option is to use a swimming pool rapid sand filter (not a diatomaceous earth filter, which would require backflushing every hour!). Rapid sand filters work quite well, although they may need to be backflushed every day, a task that is easily accomplished with a built-in backflushing valve. A major drawback to these units, however, is that they are unsightly and incredibly noisy.
You might consider building your own mechanical filter, which can be done quite simply. Sink a large plastic garbage pail into the ground next to your pond, or build a small secondary pond next to your main pond to act as a settling basin. Using either submerged pipe or siphon tubes, bring the pond water into the settling basin or sunken pail, where it can pass through a series of mechanical screens. These screens can be made from commercial brushes, upholstery stuffing meshed between grids, foam sheets used for furnace air filters or other suitable filtering materials. These screens are placed between the inlet and outlet of the filter. As water passes through the screens, particulate matter is either captured or it settles to the bottom of the filter. The pump for the filter should be capable of circulating at least one-half complete pond volume each hour.
Water exiting the mechanical filter should feed directly into the biological filter. There are many ways to design a biological filter, but the basic principle is to provide lots of surface area to support an adequate population of nitrifying bacteria. The media you choose to use in the filter will directly affect the efficiency, maintenance and size of the filter. My first preference is for large plastic hollow spheres (often seen in the trickle filters of marine reef aquariums) because they offer a large surface area per unit of volume but have large void spaces that allow solids to pass through without clogging or channeling. My second choice is lava rock that is 1 to 1½ inches in size. Gravel that is 3/8 to 1/2 inch in size is commonly used, but clogs readily and is a pain to clean.
The filter size required depends largely on the fish load in your pond. Until you finalize your plans for the pond you can't determine the appropriate stocking levels, but as an example, for a 2500-gallon pond I would strongly suggest limiting the population to six or seven koi of no more than 12 inches in length, or two or three fish of 24 inches.
There are many variables that affect filter design, but you do not need to concern yourself with them as long as the volume of media in the filter is sufficient. For a 2500-gallon pond this would be about 16 cubic feet, and you would need to adhere to the fish load recommendations above. The flow rate through this filter should also be at least one-half pond volume per hour.
There are many commercial biofilters on the market today. They are all equally effective, but the prices being asked are quite high. If you're a bit handy you can construct a much more effective biofilter yourself, and for substantially less money.