I'm not sure what the answer is but it doesn't seem reasonable at all to me that CO2 concentration is lower at the top of the tank. It's the lower regions of the tank that suffer BBA most frequently and in deep tanks it's the carpet plants that appear to be most limited. Any CO2 mist or bubbles tend to rise as they exit the injector so it seems to me that if anything, it's the upper regions which would tend to have a higher concentration. It also seems that if a fish gulps a bubble of air and passes it over the gills, that air bubble would tend to have a lower CO2 concentration than it's blood facilitating the osmosis of the CO2 from blood to bubble. I suppose it could be crudely tested by stacking an array of drop checkers vertically and observing the color change profile of each.

Cheers,