Well... did a full breakdown cleaning of the cobras this weekend and I lost the feather tank. 
Dang tank would not come off and eventually cracked.  The o-rings swelled under a particular juice I was using - Burmani.
So off to order some spares today and what is this I find... 
NOOOOOOOO!  You are out of tanks. 
I'll be ordering as soon as they come in.  Got to order a few backups too.  Luckily, I still had the original tank, but worried about loosing that one down the road - kinda saving it as a collector's piece.
I tried the other sized o-rings with the glass (11.11mm(ID) x 1.78mm thick - Also known as BS806 ), but it wouldn't seal up well enough and air would go past everytime I took a draw/minor amount of juice leakage.
Maybe time to find a 11mm(ID) x 1.9mm thick o-ring.
Edit:  Just a testament to Phiniac's original glass tanks.  They made em to last, I've yet to break one, even when dropping it from 5 feet up onto concrete more than once -- and it was one with glass decorations on the outside which hit square on the concrete seam.
Too bad Phiniac is not allowed to re-design the glass using their manufacturing specs for the cobra.  I would so buy them.
Notice how every other RBA tank out there breaks, and breaks often and usually when being removed for cleaning.  *sigh*