Noone will agree with this, and it's certainly not permanent (which is why I like it) but latex paint does the trick. Just dont get it wet. I've done it twice and glad I used latex since removal is REALLY easy, but not so easy that it peels or comes off on its own. If the back of your tank is against a wall where water or anything bumping it isnt a problem, then it works great. When it's dry it's hard to get it off using a razor but if you rub water over the surface then you can peel it off in a single sheet.