There was a "cheesy background" thread a while back. A lot of folks mentioned using posterboard. Right now I am using plain old cardboard (that was wrapped around the tank when I bought it, cut to fit) and I plan on getting some black posterboard when I get around to it. I imagine the paint looks marginally better, since it's right against the glass, but my tank is up and running so buying the paint then painting would be a hassle.