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I am not sure that I would chance it....boiling it first is no guarantee against it having toxic componants that could leach into the tank water as it deteriorates (likely to do so since it was not originally intended for submerged use).
I'd have to agree with Toirtis. I'd be concerned about what would come out of it. Especially paint or the flourescent product that used to make it glow. You can buy fake skulls for aquarium use at the lfs.