and also dangerous! very unsafeScuba and CO2 tanks have different fittings in the US and you can't fill one with the other. You may have to modify the tank to get it filled.
Selling the tanks is a better, and much safer course of action. It would be embarrassing if someone borrowed your CO2 filled Scuba tank for a dive.
it is probably better to just buy a dedicated tank designed for use with CO2
don't skimp on safety for not only yourself, but others