Hoppy –

I didn’t see your post (must have been writing mine when your’s was added) and I wish I had. It would have saved me from a bad assumption and mistake… You’re correct, I was assuming that the water in the diffuser was approximately in equilibrium with the water in that tank which is only true if water continuously passes in and out of the diffuser, not a good assumption in this case. I wasn’t looking at the big picture and I know better than to do that…

As you pointed out, if there is no continuous transfer of water in and out of the diffuser the water inside the diffuser will saturate with CO2 at which point no more gas will dissolve into it. End result – the spiral would be an expensive (but neat) bubble counter if it actually worked unless they’ve designed a way for the water to flow in and out of the internals of the diffuser, and it doesn’t look like they have.

I’m not sure why the spiral isn’t working when full of water. Maybe they made the hole in the side too big so the bubbles have an easier time going through the hole rather than traveling up the spiral. Either way, it sounds like it probably doesn’t matter much other than for esthetics.