paintball tanks can be regulated. I used to play paintball there are plenty of paintball regulators out there most bring the pressure down to around 800psi you would need another regulator to bring the pressure down further. Although I'm sure if you searched online someone somewhere has put together a paintball regulator system for aquarium use.
detail on how to build coming soon.
the fittings parts are expensive, have to get the higher standard parts to avoid trouble. a system cost about $80.