ONE MAJOR CAUTION: if it was used to fill paintball canisters, and he didn't invert the tank (mount it upside down) to fill, then that means it has a siphon tube - a dip tube going to the bottom of tank, siphoning liquid co2 through the valve. this is how those places fill the paintball tanks with liquid co2.
do NOT use a siphon tank on a regulator, it will slowly but surely destroy it, and maybe not even that slowly. it will literally freeze the regulator, and more importantly, the diaphragm.
go to a welding supply shop, and swap it for a regular, non-siphon tank, or pay them to take out the dip tube and replace the valve.
or, you can use the tank upside down to pull gas into the regulator, but this could cause issues in itself.