Fluval U Series Underwater Filters provide complete and efficient 3-stage filtration, including BIOMAX biological filtering (U2, U3 and U4 models), to create superior aquarium water quality Fluval U Filters also include a convenient flip-top lid that allows quick and easy access to the filter cartridge for aintenance or replacement including Fluval's Clean & Clear cartridge (A499). Packed with special high performance resins, Clean & Clear is proven to reduce phosphate and nitrate while preventing the spread of algae, providing you with a cleaner, clearer aquarium and healthier fish for longer intervals. The Fluval U2 Underwater Filter is ideal for use in fresh, marine or reptile environments. It comes complete with the Adjustable 3-Way Flow Control feature, 2 Foam Pads, 2 Poly/Carbon Cartridges, and BIOMAX Biological Rings The U2 is suitable for aquariums from 45 to 110 L (12 to 30 U.S. gal.). Other sizes available: -A465 Fluval U1 Underwater Filter, for aquariums up 55 L (15 U.S. gal.) -A475 Fluval U3 Underwater Filter, for aquariums from 90 to 150 L (24 to 40 U.S. gal.) -A480 Fluval U4 Underwater Filter, for aquariums from 130 to 240 L (34 to 65 U.S. gal.)