Tank Valve Cap
Keep one of these on the top of your tank to keep the sand and dirt off your o-ring. It is also used as the universal sign of a full tank - cap on - full, cap off - needs filling.
Normally selling for about $2.50, we've got them under a buck since it's yet another product we covered with blatant advertising for our website. And we'll even toss in the piece of string at no additional charge!
Tank Valve Cap - Accessories: Miscellaneous