Oceanvisions self adhesive background !!!! IMO the best available as far as a good looking background, that actually sticks to the tank and stays there. I've tried paint, I've tried prints, I've tried posterboard, I've tried plain plastic backgrounds, I've tried mineral oil, petroleum jelly, spray adhesive etc, etc. The oceanvisions stuff is flat out awesome. I spray it with glass cleaner place it and squeegie it until all bubbles are gone. The glass cleaner will go away and the background will be stuck tight and look great.
As far as thermometer, put a stick on away from the heater if you want, in a handy to read spot, and then buy a $3 floating thermometer and let it float around wen you really want to check things close.
dave