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The best thing to do is find a vice big enough to fit the tank, and put it in there nice and tight, untill you here it cracking, then swing on the reg with a nice big pipe wrench. This will make sure that you break the seal...then unwind it as fast as you can - when you get to the end, make sure that you are not inline with the reg as the remaining pressure will blow the reg right out of your hand.
Oh hang on - I think thats how not to do it....
If you are unsure of the correct way to do it - Thats a sign to get a pro shop to do it - H-Pac etc....
Do not put the tank in a vice - ever, not even losely! (Never!)