My 5lb co2 tank ran out after 2 uses. The 1/6 keg didn't even get kicked. Do u i need to turn the shut off to off to stop flow of co2 so it doesn't run out? Or do I have a leak?
You have a leak. As long as your connections and lines and kegs are sealed up tight, you don't have to turn off the main valve because there's nowhere for the gas to go. You can spray Star San (or other bubbly liquid) on the various seals to check for leaks.
Having lost a 10 lb. tank of CO2 to a leak myself, I always turn off my main valve when I'm not serving beer. This helps me to identify leaks that I couldn't find otherwise (e.g. a leaky post O-ring) because the pressure in my regulator will drop as the gas leaks out. If I turn on the gas and hear a significant amount flowing into the kegs then I know I have a leak somewhere.
The most common culprit is a bad oring on the keg or a worn ball / pin lock attachment.
If you hear gas when rocking or turning a lock, it can't be trusted.
If your regulator attached to the co2 tank with a crush gasket not an oring, these are a one time use and need replaced once the seat has been broken. Because they form into the micro imperfections of the tank and regulator and then retain the impressions causing leaks if reused. Crush types are either paper or hard plastic, they say the plastic can be reused but I've not had much success so I changed to nitril orings.