Fluval U4 Poly/Carbon Cartridges for use with Fluval U4 Underwater Filters features two sides for 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. For freshwater, marine or reptile environments. Replace every 2 to 4 weeks or when flow rate is noticeably reduced. Replace one at a time, never together. Shipping weight: 1/6 lb.