Hagen Fluval Cycle Biological Concentrated Booster (formerly known as Nutrafin Cycle) is a responsive biological aquarium supplement that ensures that all aquariums are immediately inoculated with a powerful team of beneficial bacteria, providing a safe biological habitat for your fish. These highly effective micro-organisms will create a biological flora that quickly metabolizes lethal ammonia and nitrite. Ammonia and nitrite are a result of fish waste, excess decomposing food and other decaying organics that naturally occur in aquariums. The use of Fluval Cycle is strongly recommended for new aquariums as they do not contain the necessary concentrations of beneficial bacteria required to metabolize the waste that fish produce. For fresh and saltwater aquariums. Treats 500 gallons.