Paint the outside of the tank not the inside. I used a slim roller and gave mine a few coats of a latex paint. The tape on backgrounds are nice but light glares off them and you don't the the feeling of depth with them. I still have a couple of tanks I haven't painted yet.
I know to paint the outside lol. But in order to get behind the tank I would need to move it (about 2in clearance behind the tank right now) which I don't have time to do right now. I'll *probably* do it during the summer months when I don't have so much college work to deal with.
That said, I am planning to collapse my 20g into the 75g as well, but I think I'll make a different thread for that.