I use Dow 1" styrofoam insulation - got a huge sheet (4' x 8' it think) for like $12 at a local home improvement store, and simply cut the sheet so it would fit snugly between the top and bottom lip of my tank. I used a seafoam green-ish color, and the labelling faces the wall so all you see is a clear bluish background in the tank. Cheap, easy to install, easily removed, and it even provides a small measure of insulation.