Quality biological filter media for aquarium canister filters Maintains ideal conditions to support large colonies of beneficial bacteria Efficient biological filtration breaks down harmful aquarium ammonia and nitrite Specially designed biological filtration media for optimum biological filtration. Made of porous polymer material, FilStar Bio-Chem Stars provide a substrate for the optimum growth of beneficial bacteria, including nitrifying bacteria that convert toxic ammonia and nitrite. Its unique structure also maximizes oxygen and water transfer to keep bacterial colonies healthy and active. Use Bio-Chem Stars for fresh or saltwater aquariums. 4 stars treat 20 gallons. 20 count.