A 1" overflows typically flows 600gph....based on that you can figure out you pump size going back to the tank. Remember that all turns including elbows, ball valves, anything other than straigt pipe will reduce gph along with the head that needs to travel. I got me a Quiet One 4000 which handles about 1075gph. Now I also used a Ball Valve for extra flow to the fuge. This gives me a nice steady flow in the sump return area.