Fluval has a complete line of underwater filters consisting of the U1, U2, U3 & U4.  These unique filters increase water movement and vital aeration while preforming superior water 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 three stage filtration leaves water crystal clear a vital benifit for a healthier aquatic environment. Bulid to include a convenient flip-top lid that allows quick and easy access to the filter cartridge for maintenance.