Ted,

IMO, I would spend ALL my money on a good AC unit for your place. That way it will cool you AND any number of tanks you may have.

My friend heated his entire POOL by using jsr's method by running his pool water thru PVC up on his roof and back again. It worked really well, so I think his idea is a good one.

You are only cooling 50 gallons and only a couple of degrees, so should work well.

Good luck.