Bio-Chem stars are a biological filtration media, ideal for use in all API Filstar XP and Rena Filstar XP Canister Filters. Bio-Chem Stars are a porous space age polymer material, which promotes optimal growth of beneficial bacteria, including nitrifying bacteria to remove toxic ammonia and nitrites. The internal pore structure allows ideal oxygen and water transfer through the star. Place stars in top basket of larger FilStar canister filters, or in most convenient area of smaller FilStar canister filters. One 20 count bag treats 100 gallons. Stars will become colonized with bacteria within several weeks.