Hi all,
I use a piece of polystyrene, and then paint it grey/green, darker at the bottom, lighter at the top, but making sure the colour is fairly splodgy and textured. I just get 2 pots of household emulsion in about the right colour when they are in the remaindered bin at B&Q etc.
When the paint has dried, I just wedge it behind the tank, this allows you to change colour should you wish, and provides some insulation. The really fat bits of extruded close-celled polystyrene you get for packaging "white goods" etc are great as the edges are more resilient than the thinner stuff and you can glue them together for bigger tanks. If you can get off-cuts from building work "Polystyrene Flooring Insulation Board" is the good stuff. This actually comes in blue as well, so you wouldn't even need to paint it.
You can see the polystyrene in the background (sorry about the cr*p photo, and I hadn't even managed to clean the glass), the tank backs onto a window, so the background looks a bit lighter in colour than it actually is.
cheers Darrel