well i'm absolutely not the most knowledgeable person on this forum, but taking a wild guess at the fact that many components of a fish tank are often fashioned from plastic (pieces of the filter, for example), I would venture to say that it is safe as long as it is properly cleaned.
as for boiling, i don't know many plastics that can withstand boiling, but if it doesn't fall apart in the hot water, you shouldn't need to worry about it dangerously decomposing in your tank.