Bio-Chem Stars are a biological filter media providing an ideal home for beneficial bacteria to colonise. These bacteria are constantly breaking down harmful Ammonia produced by the fish into less harmful toxins such as Nitrate. Rena\'s NitraZorb pouch will then remove this level. This media is Rena\'s most efficient media for biological filtration.One pouch contains roughly 20 stars, the stars have four times more bacteria capacity than standard ceramic media. Use one star per 20 litres (5 gallons)Can be used in an external or internal filter or sump