The expenses of paintball can add up quickly!  Constantly re-filling air tanks, paintballs, paintball field/park fees make regular paintballing a bit expensive at times.  CO2 tank O-rings are an essential piece of gear; without them, there is no seal between your tank and paintball gun and air will leak from your tank.  If you aren't careful, you can go through o-rings like candy and this can add up expenses over time.  O-rings are generally not expensive but if you have to go through a lot of them, it can add up to a lot of money if you play a lot.

One way CO2 tank o-rings get damaged is by unscrewing your tank half way, then changing your mind and screwing back in.  This can cause the threads within the paintball guns' tank adaptor to slice the tank's o-ring.  When you decide to remove your CO2 tank, the tank should always be completely removed from the adapter before it's re-inserted, preventing cross-threading and cut o-rings. 

Perhaps the most common cause of damaged CO2 tank o-rings occurs when the CO2 tank is removed from the user's paintball marker.  Most paintball players simply unscrew the CO2 tank from the paintball gun, allowing all the air in the paintball gun to spill out and freeze the tank o-ring causing it to swell.  The tank adaptor's threads often cut the swollen o-ring as the tank is unscrewed, ruining the o-ring and preventing that tank's use until you can replace with a new o-ring.  The way to avoid this is... once you've unloaded all the ammo from your gun, fire your marker slowly but continuously while you're unscrewing the tank.  This will slowly release air from the gun through the barrel rather than rushing out at the tank connection.

It doesn't take much to keep a CO2 tank o-ring in good shape for months allowing you to save a few dollars here and there.  It also doesn't hurt to stock up on tank o-rings so a broken one won't stop your day.