You are right. High pH water about 8 and higher have almost no free CO2. Here is a paper that says:
"Above pH 8, the proportion of free CO2 drops below 2-5% and species occuring in these waters would require flow replenishment or use of alternative sources of inorganic carbon(Sheath and Hambrook, 1990)"
Freshwater algae are among the most diverse and ubiquitous organisms on earth. They occupy an enormous range of ecological conditions from lakes and rivers to acidic peat swamps, inland saline lakes, snow and ice, damp soils, wetlands, desert soils, wastewater treatment plants, and are symbionts...
books.google.ie
No, sorry, high pH water has nothing to do with the free CO2 in water. This false doctrine persists despite all the obvious evidence and information to the contrary.
Gases typically follow Henrys Law, which states that at a given temperature the solubility of a gas in a solvent is a function of the partial pressure of that gas at which the solvent is exposed to. A corollary is that the solubility of the gas depends strictly on pressure, temperature and salinity.
I clicked on your link but could not find the page containing this quoted statement. In any case, Robert Sheath could not possibly have meant that pH affects free CO2. If so, he was in serious error or his editor misinterpreted his statement.
You would have been better off to have linked to the very obvious, such as in page 3 of W. Konche's
Biophysics and Physiology of Carbon Dioxide
Which clearly states in the Chapter 1 Introduction:
"The Gas CO2 is quite soluble in water which more than 99% exists as dissolved gas and less than 1% as Carbonic acid H2CO3, which partially disassociates to give (H+), (HCO3-) and (CO3--)."
This is unequivocal and there is no caveat regarding pH at 8.0 or above .
Lets look at an easier to understand source:
Understanding Henry's Law
The first major paragraph states:
Carbon Dioxide determines the pH of water
First things first: technically speaking, the concentration of Hydrogen (H+) ions determines the pH. But in practice, there's something easier to conceptualize: the amount of carbon dioxide (CO2) in solution also determines the pH of the water. The most common source of acidity in water is dissolved CO2, so
the more CO2 in the water, the lower the pH. This is because when CO2 comes aqueous in water,
a small portion of it becomes carbonic acid (H2CO3).
The "small portion" being referred to is that "...less than 1%..." W. Konche referred to.
Scrolling a little further down we see the ubiquitous chart that causes all the confusion. But this site gives a better interpretation and calls the red line Carbonic acid instead of calling it CO2 as so many other charts label it. Again, this is a proxy for the "...less than 1%..." of the CO2 that enters the Carbonic acid equation.
The misinterpretation of this chart and of the Carbonic acid equilibrium equation has given rise to all the misinformation that litters the internet regarding how supposedly more difficult it is to dissolve CO2 in hard, high pH water versus soft, low pH water. Nothing could be farther from the truth.
Cheers,
... When you have time I will probably need some advice on dosing. It seems to me that the recommended dosing (~18 ml weekly for a 150 L tank) might be too lean especially for a very densely planted tank... If I double down (36 ml weekly - perhaps split in 3 weekly doses) I get something like this (courtesy of Rotalabutterfly):
