Do you have a single overflow or more? Is this a prepdrilled tank?
Every overfllow will drain x gallons/min. You want to match a pump to that rating withouht exceeding it. To do that you need to account for the head height (distance from pump to where the return hose enters the tank.
That pump is rated at 850 gph
at 10'; 825 @ 11'; 750@ 12'; 650@ 13' . . .
You need to add a loss of 100gph for every foot of horitontal run (if you have 2 overflows) and count every 90 degree elbow as 2' of loss.