A 1/2" or 3/4" drain will not support 700 gph. A 1" hole supports 600 gph max, so a 3/4 probably some where under 500 gph. I know that on one of my tanks, I had a 3/4" drain and a Mag 9 with 4ft of head was just too much. A mag 7 with 4ft of head was just about right, just a little too much, unless you use a valve to back it off a touch.