Something to note about painting the background: the longer the paint is on the tank, the harder it is to remove afterwards, so be sure you are putting something on there that you REALLY want to keep. Anything beyond a year can be a challenge to remove, even with a razor blade.
Our store has painted backgrounds, and whenever we needed to change it, anything that had been on there for more than 1 yr was real work to remove.
Something else to note: If using a store bought background, an easy way to stick it to the back is with a few beads of aquarium silicone. It's waterproof, it sticks and sets fast, and is easy to peel off later if you wish to remove it, change it, etc. Put one bead in each of the top 2 corners, wait til they take hold (a few minutes) and then gently smooth it out and add a bead to each of the other 2 corners. It dries clear, so you won't see it, just don't use too much... a tiny bead is all that is needed.
One more comment... If you use different shades/colors of paint and a textured roller to apply it, work in a few stages to allow each to dry, you can create some really nice effects for a background so it's not just a solid color. The roller won't leave brush marks in the paint (most brushes will), and the texture tends to help the background blend in with the plants and decorations better.
On our Goldfish
system at the store, our boss had gold japaneses lettering done at a printing company. It was a gold foil, and they cut out the letters for him... he laid these on the back of the tanks and then sealed over them with a roller and coat of red paint. The end result was aswesome!!! I'll see if I can get some pictures the next time I go in (at the end of the week).
Hope this helps!