Can i fill my paintball co2 tank with co2 gas instead of co2 liquid?

if your not having fish in it you could put some ammonia in as well for fertilizer maybe even seal the top of the tank and with a tube from the top to the airpump to the airstone and just recirculate the gases

Yes, but why would you want to? The only difference between saturated fluid CO2, like what is used for paintball guns, and gaseous CO2, is pressure level. CO2 in a paintball tank is a liquid because it is compressed to hundreds of atmospheres of pressure, and expands to a gas as soon as the pressure is released. All paintball guns run on CO2 gas. A CO2 tank with only gaseous CO2 in it won't even provide a single shot worth of pressure to a paintball gun. As for the fish, you're better off using fertilizer on the plants.

learn about gases and compression pressure and force short answer yes the valve and pressure regulation is careful engineering