Put it in a vice like Mikey said. If you dont want to scratch your tank wrap it in some rubber. MAKE SURE IT IS EMPTY and use some knippex pliers to undo the reg. (if you havent got these get some good grips and wrap a cloth around the reg to stop scratching)
It shouldnt be loctited (heard some are but never found any I have done to be)
Twist and it will undo!
Like I said though, MAKE SURE ITS EMPTY!!!
Hope this helps
Nige