I have the same tank and I use a mag 12 and run it wide open. I heard others use a mag 18 though. A consideration is head heigth and whether you have a straight shot back to the tank. My 12 sits in a sump directly below so the head is about 4' or so. I've teed off the 12 to feed both returns. It may not be optimum but it does work well for me.
Cheers