Use flat black paint and the most important thing you need to do is "scuff" the surface of the back of you tank or the paint will not stick.
Use super, super, super, fine wet/dry sand paper (a minium of 600 grit) or super, super, super fine steel wool. You don't want to scratch the glass, just scuff the surface to give the paint something to grab onto. As you know, glass is very smooth and if you don't do this, it will just peel/flake off, especially when using it by water. I would NOT use spray paint however; I would roll/brush it on. Spray paint doesn't stick that well on smooth surfaces and think about if you ever need to touch it up. All you have to do is reach behind the tank with a brush, roller, Q-tip, finger, ? and put it on. With spraying, you'd have to mask off things, shut down your pumps, filters, or ???? for a good amount of time, plus spraying indoors?
Yuck! Get one of those small "Rustoleum" containers. Not a quart, but even smaller. They sell them by the stains and they're only 4 or 5 bucks. Anyway, that's my advice...good luck
