Using my non-scientist logic on this question: CO2 is freely able to move around in water it is dissolved in. It doesn't float, because it doesn't displace any water. However, at the water surface the CO2 is always leaving the water and reentering the water at the same time, with the process always favoring movement towards the substance having the lowest concentration of CO2, which will be the atmosphere. Thus, the surface layer will always tend to be depleted, causing a gradient to be built up in the tank, with the surface having the lowest concentration of CO2. If we have good water circulation it will only be the surface thin layer that will be low in CO2. But, the bottom of the tank, even with good water circulation will have a stagnant layer, because the circulation will always be laminar flow, not turbulent flow. That layer will get minimal water exchange with higher levels, and might well be short of CO2 as a result.