One more question ... is a mag 18 for a closed loop on a tank that size overkill for circulation ?

nope not as long as you set it up right.

Steve

If you use 3/4" fittings and pipe it should be just right with 4 returns. This is what I done in my 90 and even with the Mag 18 my flow is a little less than it should be. So i'm going to put 1/2" bushings in the return outlets to get a little more pressure.