I've recently purchased a 100 gal. tank and needed some advise on painting the back of the tank or going with nothing at all. the tanks against a wall ?
Black paint either spray can, brush or roll will work equally well as will enamel, lacquar, latex....., as will gloss, semi gloss, satin, or flat. Blue is also quite commonly used.
I have also used the real dark window tint as another poster stated, and it too looks fine.