In natural systems, nitrogen compounds are readily removed from the fish's habitat. In the pond, this is accomplished by the filtration system. There are three basic methods of filtration: mechanical, chemical and biological.
Mechanical filtration physically removes suspended particles from the water by passing it through a fine filter medium, which sifts out the particles. Chemical filtration involves the chemical treatment of water to remove toxic substances. Biological filtration utilizes the nitrogen cycle to remove toxic compounds from the water and is essential to every fish pond.