The Top Fin Retreat Filter Cartridges RF-S are a set of three cartridges designed for use in aquariums and fish tanks. These cartridges are specifically made by the brand Top Fin and are perfect for maintaining clean and healthy water in your pet's tank. Each cartridge is designed to last for one month, providing a 3-month supply of filtration for your aquarium. This product is for Desktop Aquariums and ensures your fish have a safe and clear environment to thrive in. Made with activated charcoal and coconut shells to absorb odors, pigments, and chlorine. Sold with free shipping.