Aquarium Systems One & Only Freshwater 250ml is composed of 100% nitrifying bacteria. It is to control ammonia and nitrite when starting a new tank, after a big water change or disease treatment. It instantly creates a bio-filter. Our nitrifying bacteria are the result of the latest scientific research and are used by public aquariums and professionals around the world to control ammonia and nitrite. One & Only can be used when starting a new tank after big water changes and disease treatments.