I have painted a number of backgrounds usually black or white. I prefer to use plati-dip, it takes a number of coats, but it looks good and is super easy to remove if you want to take it off.
I have also painted the bottom of the tank with sand texture which, in my opinion, is a cool option for a bare bottom tank.
I have also used poster board, paper, and corrugated plastic taped to the back of the tank. The plastic is nice because its waterproof.
I really love my backgrounds from Universal Rock. . . but they are a bit pricey and you have to drain the tank and silicone them in. . . so not the best option for everyone.