Tom is pursuing a membrane type drop checker head on a pH probe so he can look at CO2 levels all over the tank with a very short response time. That will be an interesting study when he gets it done. My bet is that CO2 levels in various parts of the tank vary all over the map from zero to 40+ ppm. And, I think good water circulation will reduce the spread of values a lot, but not reduce the spread entirely. I'm basing this on....ah....some very careful calculations using advanced calculus.