Or you can have the techs at the CO2 place put a "Siphon tube" in the tank. Basicallly this is a tube that goes to the bottom of the tank and draws off liquid only.
In terms of filling paintball tanks, if you are uncertain of the process here it is.
1) Weigh the tank to be filled and zero your scale.
2) Introduce a small amount of liquid into the tank to be filled , and then vent if off quickly to cool off the destination tank.
3) Fill the tank to the desired WEIGHT.
Also, always check hydrotest dates and check to see if the pin valve assembly is tight and secure.