polymer clay is basicly a mixture of pvc plastic (same as the pipes) and a plasticizer. the plasticizer interferes with the polymerization of the pvc weakening the bonds making it maliable.
When heated. the plasticizer breaks down or is inhibited by the heat (im not sure) allowing the pvc polymer to cross link again or solidify. this plasticizer can be transfered either to a piece of paper (stiffens the clay) or to a pvc pipe. I was rolling the clay with a scrap piece of pvc and the i was supprised when the surface started softening and scraping off.
Like pvc pipe do NOT overheat or it will burn causing poisonous chlorine gas.
MSDS can usually be found on the internet through google, etc. Sometimes you have to dig a bit but a "<brand name> msds" should get you on right track.
plasticizers are typically BAD BAD BAD when taking about fish, so knowing what i know I wouldn't trust them unless faced with more info.
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on second thought, PLEASE let us know what you find out. I just had a wicked idea of a nice ornament for the Dwarf puffer. a snail dispenser. a cup with multiple small passages out that allows the snails to crawl out slowly and randomly so the puffer doesn't gorge on them.