Ok,
That last post was so mangled that I couldn't really understand most of it, but from what I could gather, it was wrong.
"Nitro" for paintball refers to "nitrogen", a common, naturally occuring gas which you couldn't use in cars, and is perfectly legal. When reffering to high pressure bottles for paintball, the terms nitro and compressed air are interchangeable (a "nitro" or nitrogen tank IS the same as a compressed air tank). However, most scuba shops only fill compressed air, and compressed air is much more readily available.