Quote Originally Posted by BigSwede View Post
Note the 4th pic down on the right. Horizontal storage appears to be acceptable, provided you stand it upright for use.

http://www.powertank.com/mounting.ideas
This quote from the web page combined with Marlin's observation worries me though:

Quote Originally Posted by www.powertank.com
The light aluminum Floor Brkt. holds the CO2 tank at the perfect angle to expel vapor yet provide a super low center of gravity for maximum stability.
Quote Originally Posted by Marlin
I keep mine standing up, never tried horizontal. I will say that it does vent off in the summer, and it smells like cigarette smoke, so no storing inside long term in the heat.
I don't plan on keeping the tank in there all the time anyway, but it will be HOT when we're out on the beach. Not trying to have everyone pass out while driving...