Pondmaster Extra Large Foam Pre-Filter for Mag-Drive and Hydro-Air PumpsPondmaster Mag Drive Pump Square Sponge Prefilter works with the Pondmaster Mag Drive Pumps. Pondmaster Foam Pre Filter helps prevent rocks, sticks, and debris from being sucked into the pump. Helps prevent rocks, sticks and debris from being sucked into the pump Prevents damage to the impeller inside the pump Screen out green algae, string hair algae, fish waste, leaves and more Pondmaster Pre filter prevents damage to the impeller inside the pump.