Better option. I'm here in the Philippines, and we have easier regulations here, filling the scuba tank is no problem for us.I would sell the scuba tanks and use the money for CO2 tank
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.