Does a C02 tank hold compressed air? If so, then no it doesnt...A CO2 tank has a valve on top simply to release the CO2 and seal it back up.A compressed air tank has to have a regulator to regulate the pressure coming out of the tank.You don't want 3000 psi of air coming out at once into your gun...that would be bad

. But installing a regulator on a c02 tank, hmm I dunno but I wouldn't recommend it.For 2 reasons, 1 safety of course and 2, by the time you buy a regulator it would probably be about as much as to buy a used steel tank designed for compressed air.