Aquarium Nitrogen

Nitrogen cycle in the aquarium

Nitrogen conversion, which is undergone by plants and microorganisms (including filters) in the aquarium, is sometimes referred to as the nitrogen cycle. Very, very few aquariums, however, actually still go through a cycle in which no interventions are necessary. The creatures in the aquarium are usually dependent on forage and nitrate reduction, therefore a regular water change is necessary.

From the gills and the urine of animals kept in the aquarium, and from the unconsumed feed, toxic ammonia is initially released. If the water is not too alkaline, the nitrogen compound is present mainly as ammonium. For the fish and invertebrates in the aquarium, this is less harmful. For plants, it even makes a valuable fertilizer.

Only high pH values and temperatures outweigh ammonia. For fish, ammonia is a strong poison. If there is highly concentrated ammonia in the water, the fish are put in the position of breathing ammonia in through their gills. They ultimately poison themselves.

The ammonium that is not consumed by plants as a nutrient is converted into nitrite through the use of micro-organisms such as Nitrosomonas bacteria, which are present both in the aquarium as well as in the filter. As with ammonia, nitrite is toxic to fish and starting values of 1 milligram per liter are already fatal.

A sudden outward and shooting about of fish in the aquarium, apathetic behavior and frantic breathing indicate a possible poisoning by nitrite. However, nitrite is converted by bacteria such as Nitrobacter, in the next step, to nitrate. This process, in which the consumption of oxygen converts ammonium to nitrite, and then to nitrate, is known as nitrification.

Nitrate is not toxic to fish. It should be kept low to avoid a (partial) denitrification which would eventually cause the re-release of nitrite, harming the body of the fish in the aquarium. Nitrate will be removed by regular, partial water changes of the aquarium water.

In addition to the nitrification still going on, the respiration of microorganisms uses nitrate as a substitute for oxygen. This ongoing process is called anaerobic denitrification. It finds little place in most aquariums and can be supported, for example, through special filters if necessary.