Providing biologically-active filtration to a new tank is no longer a problem with Cycle! This super-concentrated formula works in freshwater and saltwater aquariums to keep fish healthy. Its all-natural formula is safe for all plants, animals or humans. Because of its biological filter-enhancing nature, it is ideal for use when new fish are added to your system, after medications are administered, or of course for new aquarium setup. Each dose works to reduce dangerous ammonia and nitrate levels in your aquarium as well.