Nitrogen cycle (aquarium)
The process by which bacteria in a filter turn toxic fish waste, ammonia, into nitrite and then into much safer nitrate.
Also: cycling a tank, biological filtration
Fish excrete ammonia, which is toxic even at low levels. In a mature aquarium, bacteria living in the filter and gravel turn ammonia into nitrite, also toxic, and other bacteria turn nitrite into nitrate, which is far less harmful and is removed by partial water changes and by plants. A new tank has none of these bacteria, so aquarists cycle it for several weeks before adding many fish. Adding too many fish to a new tank at once often kills them, a problem known as new tank syndrome.