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. The complete line, consisting of Fluval U1, U2, U3 and U4 Underwater Filters, includes many practical and versatile features that make them ideal for application in fresh, marine and reptile environments. Fluval U series filters provide the option of setting the flow output in 3 different positions, a key benchmark in underwater filtration. The Adjustable 3- Way Flow Control feature (U2, U3 and U4 models) lets you position the water flow output to meet your specific needs. The Top Output setting provides maximum circulation and oxygenation, the Integrated Spray Bar setting provides a gentle, even flow for planted aquariums, and the Bottom Output setting is for deepwater agitation. Fluval U Filters also include a convenient flip-top lid that allows quick and easy access to the filter cartridge