Tom,

To answer the first question, if by "hard water" you mean the disolved calcium and magnesium cations (aka GH), I would expect the amount of CO2 to be the same in each glass.

The carbonate anions (aka KH) are better referred to as alkalinity, not hardness. But you know all that stuff.

Given that, the second question doesn't seem to be related to the first, but a few plants can use KH directly (but I think they'd rather not, given their druthers.)

So, to rephrase my question, if there was one glass on a table that contained water with a pH of 8.0 (and a GH of whatever), wouldn't it would contain relatively little CO2?

If another glass contained water with a pH of 8.0, a GH of whatever, and a KH of 10, a chart at The Krib shows that it would contain 3 mg/liter of CO2. http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html

So I would surmise that only a small number of plants could be cultured at such a high pH, those few that could use the KH as a carbon source, such as vallisneria, sagittaria, elodea, and ceratophyllum. Right?

Thanks for the continuing turoring!

Bill