Many people use cheap acrylic enamels on their tanks, like Krylon or Rustoleum. Fine for glass. But for acrylic tanks, you may have a problem as anything that can remove the paint may cloud or craze the tank.
Rickey's right, try denatured alcohol first on a hidden area.
Hmm...I wonder of an acrylic tank maker would have a better answer.
i actually have had contacted wayde from atm about how to deal with these things along with large scratches in acrylic. wet sanding with a very very fine grit of sand paper like 500 or 800 grit sand paper and then using a buffer and acrylic polish. only problem is if the color was actually in the acrylic itself. this can be done to the whole tank to make it look brand new again. too much sanding can actually weaken the tank but for your purpose this shouldnt be worried about.
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