From the first day that I saw this, I thought refilling the bottle would be effortless...... Its more difficult than it needs to be..... *IMO..... I personally wish that the bottom sqounk hole ring (that's part of the mod) should have been incorporated Into the door, so that when you take the door off.... the bottle is free where you are able to unscrew it... You can't unscrew the bottle, it has to be pulled out to refill. The Bottle sits firmly in place though.
I always unscrew the bottle. You need to take the bottle out of the mod (the tygon tube will bend), and then you can unscrew it. I never had to pull off the bottle to refill. If the bottle is sitting straight in the mod, then it would be more difficult to unscrew.