Fluval U Series Filter Poly / Carbon Cartridges have two sides for the most effective filtration. The U Series Cartridges is a polyester filter pad which filters out fine debris and contaminants and a carbon side which filters out harmful liquid substances, colorants and odors. Stage 2 Poly / Carbon replacement cartridge Improves water clarity, traps debris and removes odor from aquarium water Dual sided cartridges provide more effective filtration