Dymax Filter Media Bags come in various sizes and two styles to easily store popular filter media mediums like bio balls and ceramic rings. Made of durable and machine-washable nylon with a non-corrosive zipper, these bags allow water to freely flow and promote the growth of beneficial bacteria, making them suitable for both freshwater and saltwater aquariums. Choose from small or medium sizes for coarse filter media, and enjoy the convenience and efficiency of these bags in your filtration system.