Sponge filters are indispensable for any hobbyist or breeder. These dense foam sponges can harbor incredible quantities of beneficial nitrifying bacteria. Use them anywhere you keep fish, one or two in small aquariums or multiples in larger tanks. Always keep extras going in your tanks, to speed up the cycling of new aquariums just by moving a mature sponge. An inexpensive remedy for overcrowding or inadequate filtration may be as simple as adding 1 or 2 Hydro-Sponges! Using a mature sponge is a very fast and easy way to setup a quarantine system.