The Fluval Poly/Carbon Cartridges offer two distinct filtering sides for thorough and effective chemical filtration, the crucial second stage in the aquarium filtration process. The polyester filter pad side effectively filters out fine debris and contaminants. The carbon side works to remove harmful liquid substances, colorants, and odors, ensuring crystal clear water. These cartridges are suitable for freshwater, saltwater, or reptile environments. For use with Fluval U3 filter models. It is recommended to replace filter cartridges once per month for optimal performance.