The Fluval Poly/Carbon Cartridges have two unique filtering sides that combine to provide thorough and effective chemical filtration, the crucial second stage in the filtration process. The polyester filter pad side filters out fine debris and contaminants. The carbon side filters out harmful liquid substances, colorants and odors. These are designed for freshwater or marine environments and are recommended to be replaced once per month. This pack includes two U2 Poly-Carb Carbon Cartridges.