Fish Aquarium

• The process
A well-balanced tank contains organisms that are able to metabolize the waste products of other aquarium residents. This process is known in the aquarium hobby as the nitrogen cycle. Bacteria known as nitrifiers (genus Nitrosomonas) metabolize nitrogen waste. Nitrifying bacteria capture ammonia from the water and metabolize it to produce nitrite. Nitrite is toxic to fish in high concentrations. Another type of bacteria, genus Nitrospira, converts nitrite into nitrate, a less toxic substance. (Nitrobacter bacteria were previously believed to fill this role. While biologically they could theoretically fill the same niche as Nitrospira, it has recently been found that Nitrobacter are not present in detectable levels in established aquaria, while Nitrospira are plentiful.) Commercial products sold as kits to "jump start" the nitrogen cycle, often still contain Nitrobacter.
In addition to bacteria, aquatic plants also eliminate nitrogen waste by metabolizing ammonia and nitrate. When plants metabolize nitrogen compounds, they remove nitrogen from the water by using it to build biomass that decays more slowly than ammonia-driven plankton already dissolved in the water.