To take a crack at your other questions:
Is it that simple? Wonder why I haven't noticed KH creep with lower CO2?
I would expect to see this, too, unless at the lower CO2 injection rate, it is being extracted by plants as fast as it is added. Actively photosynthesizing plants lower carbonate hardness.
And when this process happens, is it taking anything important from the water, like CO2 or Calcium? IOW, the CO2 that converted into sodium carbonate, is that not available to the plants? Is it taking anything else hostage?
The carbon dioxide being absorbed is being converted to bicarbonate and carbonate by reacting with the hydroxide in the water. Bicarbonate is bioavailable and so this not a problem. Carbonate will not be too important if your pH is below 8 or so. At 8.4 it is somewhat important, and I haven't found a good online article on its bioavailability, but I suspect this is also fairly good. So that shouldn't be an issue.
On the other hand, if the carbonate gets high enough, it can start precipitating out calcium as calcium carbonate -- the reverse process to adding crushed coral to a tank to increase carbonate hardness. Keep the pH below 8 and that should not be a problem. What is your target pH in the tank?
Will it convert back sometime after CO2 has been turned off?[/quote]
Some will. Not all at that tap pH; you've got enough excess of cations over "permanent" anions to keep some of the CO2 in the water. It would be useful to try the experiment, though, just to see if anything else weird is going on. Take a sample of your tank water and aerate it with another air stone, and see if the carbonate hardness drops any.
And does this mean there is a bunch of Sodium in my tank?..
It certainly seems like there is some
kind of alkali being added to your water, and the only other likely candidate is lime, CaOH. Which would increase general hardness, which seems unlikely given your tap dGH.
Sodium is a touch tricky to test for, but there are two possibilities. You could boil down a liter of your water and see how much solid residue is left. If you got access to a laboratory scale, you can even weigh it and immediately have total dissolved solids. (No, I don't have access to a laboratory scale, either, unless I cash in a lot of "good boy" chips with my wife the biochemist.) You can also taste-test the residue to see if it's noticeably salty; lime does not taste salty.
Or you could try a flame test. If you can fix up some kind of metal loop -- something inert like aluminum is best, if you don't have nichrome handy (see above: good boy chips) -- you can dip this in your tank water and stick it in a blue flame; a Bunsen burner or even just a gas cooking flame will work. If it turns yellow, sodium is present. You can try boiling down the sample of water first to concentrate the sodium if it's present and make the test more sensitive. Alas, neither the taste test nor flame test is a quantitative test, and sodium is ubiquitous.
If this is not what's happening, can you think of any other conditions that would create high pH, low KH water?
You'd need a chemistry in which [H+] is low and [HCO3-] + [CO3--] are also low. That's actually hard to do, since the relationship
[H+][HCO3-]/[H2CO3] = k
has to be satisfied at equilibrium. For a given carbonic acid content, raising pH and lowering HCO3- simultaneously is just not possible. (Remember that high pH = low [H+]). The carbonic acid content must be lowered; and since that
depends almost entirely on temperature and on rate of CO2 injection, there just aren't any other knobs to twiddle. (I'm ignoring CO3-- because the same ideas apply and it's not important below pH 8.)