Just be careful if you have any fish in there large enough to eat the Bubble-bits that come off (depending what type of polystyrene it is) most polystyrene is nicely squished and heated so to make a flat sheet, but stuff like packing polystyrene is made of bubbles of polystyrene simply heated together.
These bubble bits have been eaten by cichlids in the past. when eaten the gut is blocked and sadly the fish tend to swell up and die. Its very sad.