Go for a painted back. IMO Krylon is the best bet and works well. PAINT THE OUTSIDE OF THE TANK NOT THE INSIDE!!! I've seen several people do this... not good.
The paint is fairly easy to remove with a razor blade in the future.
The only other alternative I'd recommend is adhesive vinyl sheets. You can get this from most sign shops and it's pretty cheap. Sticks on well (some tricks to putting it on though. PM me if you want to got this route) and it is more resistant to scratching than the paint. It's fairly easy to remove with a little elbow grease.
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