I'm new to kegging. I just finished my first keg and was cleaning everything out to fill and get the next batch going. When I was setting everything back up, i noticed my CO2 tank was empty. From what I have seen a 5 pound tank should last longer than 5 gallons of beer, right? It's all new equipment and don't know of any leaks. Any ideas what to do/check? I had my pressure set at 12 PSI for serving. That's good for an ale, right? I'm supposed to leave the CO2 open all the time or do I turn it off when it's carbed up and being served?