Try Krylon paint, i've heard they make a type thats safe for aquariums(when dried properly)never used it myself but i was gonna look into buying some for my 55 hex to paint the background. Gonna bookmark this thread. Im pretty interested in this myself.

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My Giant Gourami has a gun and refuses to take his meds.

Check out my tanks:
http://www.youtube.com/watch?v=dyOK8Ymd9l8