In most aquariums, the biological cycle is started by introducing fish into the aquarium. The fish excrete ammonia, which acts as a food source for nitrifying bacteria. Nitrifying bacteria convert ammonia into nitrite and then nitrite into nitrate. Fritz Ammonium Chloride (NH4Cl) can be used as a source of ammonia to initiate the growth of nitrifying bacteria without the presence of animal life. Fish should not be present while using ammonium chloride.