Rosewood Pure Aquarium is specifically for use in freshwater aquariums to help achieve crystal clear and healthy water and reduce filter maintenance. The beneficial bacteria and enzymes held within each #gel like# ball breaks down Ammonia and Nitrite whilst cleaning up organic waste when added to new or existing filters. It is recommended to use 1 or 2 balls per 25 litres of water on a weekly basis. A single tub, containing 50 balls, will typically last four months when used in a 60 litre aquarium.