FX 480 carbon tank lost all of it's air when I removed it from the gun.

Had a bad day/night I charged up my little 13cu tank and when I purged the line it just emptied all the air out the purge valve on the compressor. So I had to put a new o-ring on the check valve. Immediately after that I filled my Impact with the 480 bottle on it. When I tried to take the bottle off to put the 580 on the 480 just completely emptied itself So what do I need to do to get the valve off of the tank so I can replace yet another o-ring?
 
I found a spanner wrench that actually fit with the exception of it has a square peg instead of round. So I put it in a vice and put the tank valve in it and wrapped a doubled over inner tube around the bottle. Stuck a pipe in the inner tube and cranked on it as hard as I could. It did not break loose so will make a better strap wrench and try again soon although what I did werqed on the last reg I remove from a tank. AHHH butane 🔥 I missed that so, a bit of heat is required.