simple co2 questions

i just got my kegerator built last week, and have a 5gallon apa that i force carbed slowly. i used the set it and forget it method. i set the regulator on my 5lb co2 tank to 11psi, and just left it on for about 5 days. the beer tastes great, and is carbed nicely. good uniformed small bubbles. i have a second keg of just water in it, that is also carbed nicely that i use for making home made sodas. my questions are number one, can i just leave the co2 on all the time now that it's fully carbed? should i turn it off now, and just turn it on whenever i want to pull a pint? my second question is, my regulator has a gauge that shows the psi it's set at, and a second gauge that shows how much is left in the co2 tank, and the second gauge shows that it's almost to the "time to refill" red portion of the gauge. the tank is in the refrigerator with the kegs, and i understand it will read a bit lower than it really is due to being cold. i don't recall what the gauge read when i first hooked it all up last week, but it was my understanding that a 5lb tank of co2 should last me for 10 or so kegs. why would it be reading so low already? i don't seem to have a leak or anything that i can find. thanks.
brian

Just leave the gas on all the time, there's no reason to turn it off.

Pressure varies with temperature, so the high-side gauge will read lower at fridge temps than at room temperature. It'll keep the same reading at any given temperature until the tank is almost empty and all is left is gaseous CO2 in the tank, at which point it will start dropping like a rock.

The red marking on the gauge is probably calibrated for room temperature. My gauge is constantly in the red with the tank in the fridge.

My 5lb co2 tank did the same...reading low in the fridge, nearly in the red. I posed a similar question and was basically told, yeah, it reads low in the fridge.

By the by, I recently turned up the fridge thermostat from 38 to 70 to keep the temperature regulated for a beer in primary. When cold, the Co2 guage was in red. When warm, the gauge red somewhere around 1k. Good luck.