The easiest paintball problem to fix and the most common problem to have.  The orings on the top of your tank wear out.  They fall off.  They get ripped, gouged, dried out, twisted dirty, etc.  A bad oring causes leaks that affect your performance.  Keep plenty on hand so your day isn't ruined by what should be an easy fix.  BEST IDEA EVER!: Use a twist tie to put several extra orings around your tank valve.  You'll always have them when you need them.