They are excellent for breeders, store tanks and small aquariums. All materials are self-contained. Dismantling and cleaning is cumbersome. When starting a new aquarium they would be introduced in small numbers along with the fish and water. It is important to remember that nitrifying bacteria attach to solid surfaces such as gravel and require oxygen to oxidize ammonia and nitrate which, in turn, they use for their energy.
Nitrogen enters the aquarium in the form of protein in food as ammonia from fish waste.
Bacteria convert fish waste and excess food into ammonia. Ammonia is converted into nitrites by the nitrifying bacteria, and nitrites are then converted to nitrates by other nitrifying bacteria. Both ammonia and nitrites are toxic to fish, while nitrates, although non-toxic, need to be removed by regular water changes to provide the optimum environment for keeping healthy fish. Remember that bacteria colonize on the surface of filter mediums or on gravel. An aquarium with a heavy planting of tall background plants gives fish a sense of security.