Like Coach said, it depends on what your overflow is producing. I use a Mag18 on my 75g. I built a tall stand which means there's more head (it has to push the water more to get it back into the tank). I went from 1200GPH overflow to 600GPH (the first one broke) and now the ball valve on my return line is closed about half way. I'm about to order a 1400 GPH overflow box, then I can open it back up.
Magdrive baby.