Lots of tape. I don't know if you can get a background to look like it's a part of the tank. I have seen stuff on here where folks have painted the back of their tank - seen some photos, looks awesome. I would do it if I wasn't so lazy. My backgrounds look kind of crappy if you look at them from the side/back of the tank, which is kind of difficult to do, but from the front it looks good, so I don't care. Sorry, I'm not much help.
Peggy