Not really quite so different as far as the average person goes. Polystyrene is one of the more common plastics, those white bread boards are probably polystyrene. In fact, most plastic things you have around are probably polystyrene. It can also be turned into a foam called expanded polystyrene(EPS) or styrofoam, just the same as polyethelyne can(EPE).
Get some of the cheap black irrigation fittings to put any bends you need in. The cheaper hose won't kink unless you're trying to twist it at too much of an angle.
Though, I haven't found the stuff I've used to kink unless I'm being ridiculous with it.
I wouldn't do anything until they start to get over 90, just opening the lid and blowing on the surface with a fan should get you an easy 10°F drop in temp.
Edit -Billaney , No need to be smart
I agree though, probably your best bet if somewhere like Mitre 10 doesn't have some vinyl letters or whatever that will work.
Does it stay at a constant 18ish degrees? If it doesn't, No you cannot just keep it without a heat like goldfish which can handle all the way up to tropical temps.
Even the most athletic person on the planet only removes somewhere around 1/5th-1/7th of the oxygen out of every breath. So, at best your breath is about 3% carbon dioxide, 18% oxygen and 78% nitrogen.
Heh, I always think it's amazing how few are in them. I usually defrost half a dozen at a time in a cup of water to get enough to give my fish a reasonable meal.