You want to match you return with the flow through rate of your overflow. If your overflow drains 600
gph then you should get a pump to match that. I have dual overflows in a 120 gallon tank that are rated for 600
gph each (total 1200
gph). One of the drain lines from the overflow to the sump has an elbow which probably decreases the flow a little so I doubt if I'm draining 1200
gph total. I also run a Mag 18 which is rated for 1800
gph at 0 head. It does 1375
gph at 4 ft and a little over 1200
gph at 5'. On the return lines there are a couple of 45 elbows and a tee going to the return. I also have a tee to divert some water to my Refugium with a ball valve to control that flow. Overall my flow rate is les.s than 1200
gph but you can see how the pump and the flow rate from the overflow match