Greg,
I was fully intending on foaming the back. I even bought the foam. but the more I stared at it...the more worried I became about seeing the top of the foam. Foam in a tank with a canopy or top looks great. But foam that is not in the water, but exposed to light can turn yellow and looks terrible! I didn't know exactly where the water line would be, and I didn't think I could make a clean edge 1-2" down the back wall....so I bagged it. It was just to risky, and that crap is permanent.