One of the most fascinating parts of pond keeping for the enthusiast is the management of the pond water quality. Water quality refers to keeping ammonia, nitrite and nitrate levels down to acceptable levels as well as controlling PH and other chemical properties such as water hardness.

In the case of ammonia and nitrite zero is the acceptable level since both are potentially very dangerous to fish life. This where a biofilter comes into use.

In every fish pond ammonia is excreted as the direct result of a fish's metabolism. The ammonia in the presence of specific naturally occurring bacteria is converted to nitrite which in turn is converted to nitrate by a second class of aerobic bacteria. Ammonia and nitrite if allowed to build up are lethal to fish whereas nitrate is much less so.

In a natural lake or pond the bacteria build up on all surfaces and the amount of fish that such a pond or lake can hold is influenced by the amount of bacteria in the system as well as available food suppy.

In a garden fish pond however the surfaces on which bacteria can build are generally insufficient and have to be artificially increased by installing a biofilter.

The absence of a biofilter in a fish pond system especially if the pond is overstocked or overfed will cause the fish in the pond to be stressed and could quite easily cause significant fish deaths.

A fish pond biofilter does the job of removing solids and particles that remain suspended in the water as well as purifying the water by oxidising ammonia to nitrite and then nitrite to nitrate. These processes taking place in the biofilter also need oxygen which is dissolved in the circulating water through the biofilter. The removal of solids by the filter is a secondary job. The primary function of a biofilter is conversion of toxic ammonia and nitrite to nitrate.

In a fish pond food eaten by the fish is partly digested and partly expressed as a waste product, which sinks to the bottom of the pond. Ammonia is excreted as part of the metabolic process. The more food that is fed then the more ammonia is excreted. Ammonia dissolved in the pond water is potentially lethal to the fish in a pond unless it is removed by biological conversion in a biofilter.

At their simplest pond biofilters are literally containers holding sponges, brushes or other media capable of trapping particles as well as a biomedium on which bacteria can grow and carry out the biological oxidation of ammonia and nitrite. More sophisticated biofilters include pressurised biofilters, vortex filters with Japanese matting, and fluidised bed biofilters. Small sized pressure filters have become very popular with fish pond keepers. The larger vortex and fluidised bed filters are generally used in large koi ponds.

The very common gravity flow biofilters are boxes which contain a biomedia substrate which in most filters is some form of plastic tubing (the best known is called Flocor), plastic balls, or other plastic shapes. The better pond biofilters will contain Alfagrog ceramic biofilter medium (Supra in the USA). These biomedia surfaces act as breeding grounds for the bacteria that do the job of purifying the pond water ... ie converting ammonia to nitrite then nitrite to nitrate.

Remember that a biofilter first and foremost is designed to purify water and the secondary purpose is to remove the solids that make water cloudy.

You will often see foam sheets in commercially available biofilters. These are there to catch large particles but also provide surface on which bacteria can grow.

By far the best means of getting large surface area into a small black box biofilter at low cost is to use porous ceramic materials like "Alfagrog" also called Supra in the USA.

For large ponds or those people wanting minimum hassle filters should consider bead type filters. In the USA the leader is GC Tek with their Aquabead range.