I used a 97cent can of flat black, works great. Doesn't need to be anything fancy, just razor blade the glass first, wipe it down with 90% alcohol, wipe it down with a microfiber cloth to get the dust off, then spray it up. 2-3 coats later and you're good to go. Of course I tape and paper the other sides and top so the over spray doesn't get in or on them. And I always lay the tank flat so the side I'm spraying is flat so there will be no runs.
Steve