Bio-Chem stars are a biological filtration media that provides substrate for optimal growth of beneficial bacteria, including nitrifying bacteria to remove toxic ammonia and nitrites. The 50-70 internal pore size allows the best 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 within several weeks.