Filters for Koi or Garden Ponds. The main use of filters on ponds, is for biological filtration. There are three main types of filtration: Biological, Mechanical and Chemical. Biological Filtration is the breakdown of toxic fish waste by living organisms to produce less toxic end products. Generally, friendly bacteria (e.g. Nitrosomas and Nitrobacteria spp.) breakdown Ammonia (which can be toxic) through Nitrite (also toxic) to NItrate (only toxic in very high levels). Partial water changes and, or de-nitrating filters (e.g. vegetable filters) are then used to prevent too high nitrate levels. Mechanical filters are designed to remove particulates; for example sediment, leaves, chelated or larger algae clumps, in the water. Removing particulates prevents them from clouding the water or otherwise being unsightly. Chemical filtration is the use of chemicals, for example Carbon or Phosphate remover, to remove chemicals or substances from the water. Usually chemical filters work by binding the unwanted substance to themselves, so that when the chemical filtration media is removed, so is the unwanted substance. Some chemical media are rechargeable, some are simply discarded once they have extracted as much as they can.