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 for maintenance or replacement.In addition, Fluval U Filters offer installation versatility: they can be installed vertically against the aquarium wall to create currents or customized flow patterns or they can be placed horizontally in shallow water tanks or to create decorative features such as waterfalls.The Fluval U1 Underwater Filter is ideal for use in fresh, marine or reptile environments. Use the U1 as a primary filter in smaller aquariums or as a supplementary filter for larger aquariums. It is also ideal for tanks with low water levels. The U1 includes an adjustable flow control and comes complete with 1 foam pad. Fluval U1 is suitable for aquariums up to 55 L (15 U.S. gal.).