Polystyrene (styrofoam) leeches chemicals at 86 F, and melts at 100-120 F, so imo, it's safe for fresh cold water and most if not all saltwater setups, but a questionable choice for many tropical systems. (That's the reason we are all advised not to heat up items in the microwave using styrofoam containers.)
Exactly which chemicals are leeched, how much and to what effect on tropical fish? I don't doubt that some people use it, perhaps with no complaints.
Could it be a trivial amount? Could it not affect fish? Absolutely. But there are still other alternatives which we know are completely inert. Large thin pieces of slate would work as well if there is a potential for something large and sharp falling against the tank bottom.
http://www.homedepot.com/b/Flooring-Tile-Natural-Stone-Tile-Slate-Tile/N-5yc1vZash2