You can always make a background out of construction paper or .. er .. shoot, I don't know what it's called, but in the lighting aisles at Home Depot or Rona or whatever, in the section with the lighting diffuser stuff, there is this stuff that looks like corrugated cardboard, but it's a lightweight plastic. I think it's in the lighting aisle anyhow, I'm not sure what it's meant for. But it comes in various colours or you could spray paint that with whatever colour you like and then tape that to the back of the tank. I've done this and several others have done this on their tanks and it works well. The best part is, if you decide you don't like it, you can take it off and replace it with something else.
