These airation stones are a bit of a waste of money, I have one stone, and it cuts about a day off of my forced carbonation time, I have multiple kegs, and well frankly having one finish early does me no good. Additionally I have not figured out how to get the stone out of the keg once it is carbonated, which in its self causes some foaming problems towards the end of the keg.
I don't know about your kegs, but not all of mine have a small maybe 2 inch metal pipe inside the gas in side, some of mine are made of plastic. I found the tubing to the stone does not attach well to the ones made of plastic. The tube to the stone attaches well to the tube made of metal with some force. I know some people have their stones set up so they can be removed, but I found the easiest thing to do was to attach the tubing to the little downspout inside the keg where they let the gas in and let the stone drape to the bottom. Essentially every time that you add CO2 to the tank, the CO2 is being added to the beer instead of being added on top of the beer. When you get to the end of the keg, it really foams up the beer inside (actually I don't know what is happening inside, but my beer comes out with more foam) as the beer level drops closer to the stone.
Hope this helps and is somewhat understandable
