The Fluval Poly/Carbon Cartridge has two 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, colourants, and odours. For freshwater, marine or reptile environments. Includes 2 Cartridges. For best results, alternate cartridge replacement every 2 weeks.