You need to twist that off with a big wrench as Sanchez says, then try using crc 'freez off' to shrink the neck of the tank. These tanks have a way of sealing themselves to the body as others have said, you need lots of lubricant and twisting to ease the neck out of the body. There's a rubber o ring on the tank that is surprisingly effective at holding it in place against the body, the more lubricant the better - I use atf and acetone 50/50 in these situations