If you look at the pics and video of my tanks, they are rippling good so there is PLENTY of O2. There is zero surface scum. Surface scum will retain CO2 and this is good if you add too little, as you add more or if the scum comes and goes as is often the case for many...........then this can gas the fish by not allowing it to degas as it gets higher ppm. As the concentration of CO2 increases in the water relative to air above, so does the flux. Same with the thickness and type of boundary layer. If this boundary layer is reduced in thickness and/or the scum is removed allowing from much better/more stable CO2/O2 exchange, then the ENTIRE SYSTEM IS FAR MORE STABLE OVERALL.
Some examples and these can be used to estimate things like sediment flow through and heating cables, UG filters etc.
J= -D * Concentration difference/distance of boundary layer
If the type of boundary layer is going from clear water with no surface scum layer to one with a scum layer, either whoile or partial, this GREATLY will effect the flux.
The distance change is still relative very small vs the two types of boundary layers.
So the equation reduces mostly to: J= -D * type of boundary layer/a set distance.
The equation predicts:
As the concentration of CO2 increases(say 10ppm to say 50 ppm), the tank with less/no scum layer will diffuse out FASTER than the tank with the layer.
As the CO2 is reduced, this difference is reduced. This is because the concentration difference between the air and the water is reduced.
This may explain why there are large differences in CO2 in tanks with higher flow and wet/dry filters vs tanks with canister filters which tend to have scum layers, visible or not ....and lower O2 levels.
Scum layers come and go and may often appear after night and then fade away after a few hours in the morning.
Some tanks have long term persistent scum layers.
These tend to have poor O2 and a few have larger O2 swings daily and higher CO2 ppm at night.