Abstract: A submersible centrifugal pump especially for aquariums includes a motor driven impeller for circulating water through the pump and a movable flow indicator visible from the exterior of the pump for visually indicating the water flow rate through the pump. The impeller is mounted on a hollow motor shaft which protrudes above the pump to provide for the passage of air down through the shaft for mixing with the water within the pump. The shaft is rotatable relative to the impeller to permit turning of a valve shutter connected thereto relative to a valve seat for controlling the water flow rate through the pump.