Aquarium Equip 2pcs Filter Zip Net Bag16.5"x12.6"- Pond Bio Ball Breeder Media Fish Tank SIZE42CM(16.5") X 32CM(12.6") The filter bag is made with corrosion-resistant materials. It is constructed of polyester net with zipper closure. When using it, put the filter medium into the net bag, and then place into the filter tank. Netting design Fish Net Bag allows free flow of water to keep oxygen levels up. It is suitable for pack filter material, such as bio ball, ceramic and ammonia remover.