Hi Martin,
In response to Victors query, when I mentioned the idea of "holistic", what I'm saying is that when you think about CO2, you have to see it as the central theme, let's say, as in a novel, for example. Everything revolves around the Carbon Cycle.
In a tank, there are more ways of getting CO2 to the plant than just turning up the injection rate, as we know. The distribution method being used is not just "can I punch the water jets to the front glass?" It's more about "can I move a large enough volume of water across the surface of the leaves in order to reduce the boundary layer thickness", or, "can I saturate the water column sufficiently to improve the CO2 partial pressure?" It can also be as simple as "can I reduce the need for higher partial pressures?"
I think I mentioned in another post that it's often the case that CO2 failures occur at the beginning of the photoperiod, and that at the tail end of the day, it's quite possible that the CO2 level is high, but by that time, the plants don't care because they have decided to do other things instead of fixing Carbon to make sugar. There is a schedule to be followed, so that Carbon delivery has to be made according to that schedule. The flow rate through the spraybars or the injection rate through the diffuser may not matter if the timing of the gas is off schedule.
That's why I thought it bizarre when you decided against adjusting the alkalinity of your water in order to more accurately measure the availability of the very thing that is responsible for the growth performance of the plants. Once you can more accurately determine the behavior of the gas then it tells you how to move forward. The pH data tells you how efficient the other things that you do are, so this information is fundamental.
Porting the gas through the filter, having an on-schedule gas delivery, being careful with the lighting, as well as flow energy and distribution patters all work together to produce the results you're looking for. The bigger the tank, the more important all these items become.
Cheers,