Painting the background of your fish tank is a great alternative to those wallpaper type backgrounds that you tape on. It is nearly impossible to keep those ready made backgrounds looking nice, but a painted background is maintenance free. Sometimes you can even paint the sides of the fish tank. This is a very effective way to keep sunlight out of your aquarium, which cuts down on algae growth on the glass.
The easiest way to paint your fish tank background is to simply spraypaint it. To do this the fish tank must be empty, and outside of your house. First mask off anywhere you don't want to get paint. I like to use 1" blue painters tape and newspaper. Remember to cover the top of the aquarium as well. I use the cheapest flat black spraypaint they sell at home depot. You could also try blue or green. I have even seen someone have blue at the top which then faded into black on the bottom of the fish tank. The key to getting a good outcome with spraypaint is to use many light coats of paint.
Just when you think you have an even coat over the whole aquarium put a flashlight inside it pointing out towards you. This will show you anywhere that the paint is not thick enough. Continue adding light coats of paint focusing on these areas until you can't see any light from the flashlight coming through the paint.
I used two cans of spray paint to paint my 15 gallon aquarium, and I did the sides and back. Overall I am very pleased, and from now on all of my fish tanks will have a painted background.