I usually use mat board (can get at framing or craft stores), it is thicker than poster board, comes in a zillion colors and some subtle patterns, and is less likely to wrinkle if it gets wet. The "real" duct tape (shiny smooth, paper backed, actually used to seal heating ducts together) works great to attach it to the top and bottom of the tank. Cut it so that it fits in between the upper and lower trim strip and tape it in place. That way it would be pretty hard to get water in between the tank and the background. The only drawback to mat board is it is a little more expensive than posterboard or a can of spray paint.