Your return pump should only supply a very small percentage of your overall flow. Use powerheads, streams, vortechs to provide most of your tank flow. Need to know where your sump is located in relation to the tank to give an actual pump suggestion because the amount of head pressure plays a critical role in picking a pump. But in general, you're looking for a pump that can supply between maybe 1,000 and 2,000gph to your tank.