Maciek,
There's something odd about the interpretation of that chart. I can't read the labels of the axes but this ought to be basically the same data we see for a dropchecker, i.e the pH/KH/CO2 relationship. The CO2/H2CO3 equilibrium constant is a function of the temperature/pressure of the water (and gas), not a function of the KH. At 25 DegC & Sea Level the number is something like 600 (the equations use the inverse of this, which is like 0.00166). What this means is that only 1/600th of the mass of dissolved CO2 actually converts to H2CO3. That means 599/600ths of it stays as CO2.
The way I look at that chart, if you have 3 cups of water, one with an equilibrium pH of 4, the second at 7 and the 3rd at pH 10 they will each have exactly the same amount of CO2 molecules dissolved in them. But the tail can't wag the dog here. I'll need to consider why the pH of first cup is at a pH of 4 and why the pH of the third cup is 10. Was the first cup driven to pH 4 by the injection of CO2? In fact were all three pH values arrived at by CO2 injection? If yes, was it the same amount of CO2 injection i.e the same amount of grams in each cup?
OK, lets say the answer is No. Let's say that I arrived at a pH of 4 by the addition of another acid in the cup of water and that the third cup was driven to pH 10 by the addition of a base. What this tells me is that in cup 1 the acids drive the equilibrium equation CO2 + H2O <=> H2CO3 to the left. But in cup 3 we have added a base so this drives the equation to the right. But a pH of 10 is completely off the chart and that's a lot of base to add. This means that a non-injected tank would have it's atmospheric dissolved CO2 completely converted to Bicarbonate/Carbonate.
We're not operating in either of those extreme regimes and on top of that we're injecting CO2. We're all operating somewhere between a pH 6-7 which means this chart would not apply.
OK, let's say the answer to the question is Yes. The pH values shown on the chart was driven by CO2 injection. Well there is no way at pH 7 that 80% of the dissolved gas is converted to Bicarbonate. That doesn't match the value given for the equilibrium constant.
This chart has got to be telling us something else. Could you help with the Polish mate?
Barr indicates that within our regime the solubility of CO2 is pretty much the same at any of our KH values. I didn't see any difference in terms of CO2 when I switched from soft water to hard water for the vast majority of the species. That's why I'm not really convinced that lower pH or lower KH makes any difference except for perhaps a few plants which are the subject of the discussion. There are just too many variables to draw that conclusion. And I'm talking about known difficult plants, A. reinikii, HC, Glosso, P. helferi - all grew better in high KH/higher pH than in low KH/lower pH water.
What I can't quantify is exactly how much more CO2 injection, if at all, did it take in the high KH tank to equal the growth performance compared to that of the low KH tank. I always try to point out that it's not even a good idea to compare tank performance at different years. I was a better plant grower by the time I decided to use tap water than when I was using soft water. So that has a tremendous effect of the comparison. Your L. cuba grows like a weed now because you are more aware of the importance of CO2/flow now than when you were using hard water. So that affects your results.
We really need to setup multiple tanks side by side and apply the growing principles equally across different water types to get good data. it's really the only way to determine the effects and the degree of effects.
Cheers,