Here's a little trick I learned. Many times, if you have a printed background and you just tape it to the back of the glass, it will appear very dark. Lay your background out on a table with the side you wish to view face up and wipe it down with a wet sponge. Then "stick" it to the back of your tank. The water will create a suction effect, almost gluing the background to the glass.
Tape one side of the background to the glass and then grab a credit/debit card or something similar and work out the air bubbles, working from the side that is taped down toward the side that is not. When you get to the other side, and you have all the air bubbles out, tape the other side down and your background will be very clear and bright through the aquarium glass.
I've found the hardest part of putting a background on our tank is simply remembering to do it BEFORE you have the tank filled, LOL
