Bjorn Although expressed as CaCO3 equivalents, at the pH ranges and gas saturation levels we are working with, the predominat ion is bicarbonate (which I guess is what you are calling hydrogencarbonate, HCO3).
Calcium carbonate is esentially insoluble (until pH <<4.0) I can't get it to melt until pH 2.0 in a reasonable length of time. But if you want to generate measurable alkalinity fast, add some baking soda (NaHCO3) to some RO water, and the math is easy.
The conversion from alkalinity as CaCO3 to bicarbonate ion is just a factor of 1.22. So "alkalinity as CaCO3)"X1.22 will give you bicarbonate ion concentration.
I know that calcium carbonate essentially is insoluble, the use of it is again just a convention. Just like presenting NPK in the fertilisers. Nobody think that N is there as nitrogen and K is not there as metallic potassium.
The below link is describing many of these aspects in detail.
http://books.google.no/books?id=ytG...epage&q=nitrogen assimilation orchids&f=false