There are billions of microorganisms present in a pond, each carrying out their own unique metabolic duties. The most significant microbes to you, the water-gardener, are the pond’s ‘friendly bacteria’. These bacteria are involved in nitrification processes.Nitrification is the conversion of ammonia (NH₃) to nitrite (NO₂), and the conversion of nitrite to nitrate (NO₃). This is an important process in the pond because ammonia, which is present in the pond from excretion by fish and the breakdown of proteins, is highly toxic. Nitrite, although less toxic than ammonia, would still render the pond completely inhospitable if levels were allowed to rise. The final product of nitrification is nitrate, which is far less poisonous and is used by pond plants as a fertilizer.The main microbes responsible for converting ammonia to nitrite are bacteria belonging to the genus Nitrosomonas, and nitrite is converted to nitrate mainly by bacteria belonging to the genus Nitrobacter.

Nitrosomonas and Nitrobacter bacteria require wet or moist surfaces to grow and reproduce, and a continuous supply of nutrients (in this case ammonia and nitrite, respectively) and oxygen to metabolise and convert the toxins. They are able to live in a fairly broad pH range, but can not handle sudden pH changes. These bacteria become dormant below 5?C, and will begin to die below 0?C. Their metabolism and reproduction increases with temperature, up to around 30?C.The bacteria will be present on all surfaces in the pond, but because the pond should be regularly cleaned, many bacteria may be removed or killed. This is the reason that pond filters contain ‘bio-media’. The bio-media in a filter is a protected area for the bacteria to live, reproduce and metabolise.

There are many types of bio-media available. The more efficient bio-media have a larger surface area, which will allow for a greater population of bacteria to be housed in the filter.The pond water should be able to pass freely through the bio-media, there shouldn’t be any ‘dead’ areas with no water movement as this can promote a process called denitrification, which is the conversion of nitrate back to ammonia by heterotrophic bacteria. Heterotrophic bacteria are only present where there is no oxygen dissolved in water around them.Most filters have a static bed of bio-media which the pond water simply passes through. The bio-media in this set-up has to be fairly dense so that it stays in place in the filter. Some filters incorporate a ‘fluid bed’ of bio-media. In this design the neutrally buoyant media is constantly agitated with air from an air pump or by the movement of water through the bio-media chamber. This continuous motion will remove any older or dead bacteria attached to the bio-media which can be taking up surface area and which may begin to mineralise. If an air pump is used it is also a direct supply of vital oxygen for the healthy bacteria.When cleaning the filter it is important that the bio-media does not come into contact with tap water as all tap water contains chlorine which will kill the bacteria. The bio-media will develop a build-up of sludge which should be removed, however, cleaning should not be too vigorous, as healthy bacteria will be removed. The best way of cleaning the filter’s bio-media is to remove it from the filter and swill it in a bucket of pond water. The bio-media chamber can be rinsed with pond water before placing the media back into the filter. Be careful not to let the media dry out, as this will kill the bacteria. As some of the cultivation will inevitably be lost, it is usually worth using a filter treatment product after cleaning.

Biological Breakdown of Ammonia.