Contains two (2) API NITRA-ZORB SIZE 6 Aquarium Canister Filter Filtration Pouch 1-Count Bag Reduces the risk of fish loss. Great for starting a new aquarium. Size 6 - Fits most filters Easy-to-use pre-dosed pouch fits most canister filters on the market. For use in freshwater aquariums. Use when starting a new aquarium and when ammonia or nitrite is detected, replace every 2 months.