www.freshwater-tropical-fish-tanks.com There are two basic cases where the fish population must be considered. Once the bacterial filter has become established strongly enough to eliminate all the ammonia and nitrite as soon as it is produced, the nitrogen cycle has become well established. At this point, the mature system can tolerate a gradual increase in the total bio-load it is able to support. Once the two bacteria strains required to eliminate these toxins are at the right density, the bio filter has become properly initiated. Once matured, the maximum fish population that a standard aquarium, with suitable and strong filtration can safely accomodate is about: Two inches per gallon.