The nitrogen cycle describes the process whereby ammonia products, which are secreted by animals as waste, are converted by bacteria to nitrite and then into nitrate. Ammonia and nitrite are highly toxic to fish in very low concentrations, so establishing the bacteria colonies that quickly convert these compounds to nitrate is crucial to creating a healthy environment for fish. Nitrate are far less toxic, and can easily be removed through periodic water changes or consumption by live plants. Most fish mortality in new tanks can be traced to the lack of an established nitrogen cycle in the tank. Fish excrete urea, which contains ammonia. In a new tank that does not have the necessary bacteria colonies, this ammonia will rapidly accumulate to the point where it is toxic to the fish. Depending on the size of the tank and the number of fish, the ammonia may become toxic within one day to a week or so.

New Tank Syndrome (NTS) describes a tank that does not have the necessary bacteria colonies, and which kills fish as a result. Every new tank must under go what is known as Cycling. During this time you must closely watch your tank and monitor the levels of ammonia, nitrite, and nitrate. By monitoring these parameters you will be able to tell when your tank has become habitable for fish, or if it is still in the Cycling phase.