Q. I am building a 2000-gallon pond. I plan to keep a dozen koi or so and am considering what kind of filter I need. I was thinking about using only a plant filter, but I was told a biological filter was an absolute requirement. Is this true?
A. The term “biological filter” is used by aquarists and pondkeepers to connote a filter specifically designed to provide a good habitat for nitrifying bacteria that detoxify ammonia. Beyond that, the term has no direct physical meaning.
A good plant filter design can make an excellent biological filter in two respects. First, the plant roots and submerged stems make an excellent substrate on which nitrifying bacteria can grow. Second, the plants themselves will remove ammonia from the water and hence act to biologically filter the water as well. Therefore, if you design the plant filter properly, you will have all the ammonia removing capacity you need.
A plant filter also makes an excellent, as well as maintenance-free mechanical filter. I would recommend a plant filter box that holds about 200 gallons of water and is about 1 foot deep. I would not use a gravel or dirt substrate, but instead place yellow iris (Iris pseudocarus) bare root in the water with some stones to hold them in place. Alternatively, floating water hyacinth will work well. In either case, the box should be filled with plants spaced 6 inches apart. Recirculate the water through the plant filter at least once every two hours.
Never, ever use plant fertilizers. The nutrients will only cause problems and they aren’t needed.