The AquaStep Pro UV Sterilizer can be used in two ways to kill disease causing organisms and clear free floating algae in your aquarium: as a hang-on unit or under your aquarium. This unit will triple the contact time of water with UV rays as the 'internal step design' directs water back and forth across the light. Other features include: an adjustable clamp to secure a snug fit on your tank or in reservoir tank, high quality UV bulb; compression coupling and gasket for creating a water tight seal around quartz sleeve and ballast cord; electronic ballast; inlet and outlet on both top and bottom of unit for different installations; free standing base and heavy duty molded construction. 15 watt model has 3/4" inlet and outlet and has a flow rate of 450 GPH. Pumps recommended to go with this unit are the Quiet One 1200 or 2200. 115V.