I use a sheet of pexi from Home Depot. Paint it however you like and tape it to the back with a few pieces of waterproof hockey shinpad tape.
You can remove it in seconds for cleaning or use a magnet cleaner without worry of scratching or ripping.
There was a recent post about better light reflection in the tank using pexi instead of paint.
http://www.canreef.com/vbulletin/sho...916#post344916