Whichever you do, make sure you verify the bps was stable for at least two days, meaning your observation ought to begin earlier so you have time to make adjustment. When I was new in CO2, I made the fatal mistake of making a last minute adjustment before I left for vacation.
I replaced with a new cylinder for fear that it would run out. But I didn't spend enough time to verify the bps was stable. The final nails in the coffin was I made one final needle valve adjustment just before I left because I wasn't happy with the bps. When I returned home 2 week later, there was a foul smell when I entered the home. 10 lb of CO2 was dumped in 2 week, all fish dead, and algae was all over. The tank had turned into a green, foul smelling fish soup as all dead bodies had disintegrated. After a complete clean up, and dosing of Excel daily for two weeks, all plants had recovered. I lost all the fish, but not a single plant which was over fed with CO2 and NH3 during my absence.
When the first gauge pressure drops to near zero, the cylinder can still last 2 to 3 weeks before running out depending on the size. Even if the cyclinder empties out during your absence, it won't kill your fish. The plants may suffer, but recoverable with remedial measures after you return home.
Originally Posted by theatermusic87
If you decide to run the tank until it's empty, prop the tank up on something so the regulator is above the water level of the tank. That way if you do run out of pressure you're not relying on a check valve only to keep the line from siphoning water into the reg and trashing it
The check valve is to prevent back siphoning. If you can't trust your check valve, why install it. To check your check valve, install it submerged inside your tank, so you can observe it.