I still see no point in using this chart in non CO2 fertilized aquariums.
Agreed, to an extent. Because CO2 will always try and maintain equilibrium with the surrounding atmosphere. Henry's Law
. So determining the CO2 levels with the chart is pointless, since CO2 levels will be at equilibrium with the atmosphere (or thereabouts).
However, CO2 - KH - pH have a relationship. If you know the value of (any) two, you can determine the third. So as above, we assume (non CO2 injection) that the equilibrium of CO2 with the atmosphere is 3ppm, and the pH is 7.3, then we can determine that the actual Karbonate hardness is around 2 dKH. Most people only try and determine CO2 with pH and KH, but you can determine pH with CO2 and KH, or KH with CO2 and pH.
However this person does not change the water on a regular basis and tries to create a blackwater setup with all kinds of leaf litter, driftwood, peat and tannins. All these acids accumulate and manage to lower the pH to 6.4. The KH remains the same, 9, so according to the chart this person now has ~CO2 107ppm. Poor fish must be gasping plants growing like crazy, though no CO2 was added. Nothing like this, in this setup the %carbonic acid which lowered the pH is minimal, the chart is totally inaccurate and useless.
In this situation, KH only remains the same if the test kit measures total alkalinity, rather then only the Karbonate hardness portion of total alkalinity. Tannis will
lower KH, since tannis will
convert carbonate and bicarbonate (Karbonate hardness) to carbonic acid.
In this situation, carbonic acid levels increase, however, the level of carbon content in the water has remained the same, since there was no net increase in carbon content, mealy a conversion between the carbon species
H2CO3 <--> HCO3 <--> CO3.
Notice how the carbon portion of the three species is identical (CO3), simply the number of attached H+ ions changes.
So even though the carbonic acid (H2CO3) level increases, any increase in CO2 dissolved in the water is minimal, since the carbon levels have essentially remained the same.
So really, it's not that the chart is inaccurate and useless, simply people do not fully understand the chemical process involved, and are misled by manufactures that test kits measure Karbonate hardness, when they do not.
Assume the KH=9 is the actual carbonate hardness. This person starts to add CO2 and cranks it too high. Now only dissolved CO2 is responsible for the change in pH. The pH drops to the same 6.4. Otos jump out of the aquarium and the Betta is happy that it belongs to Osphronemidae family.
Where we inject CO2 into the water column, only a small portion of CO2 converts to carbonic acid
, so KH doesn't change because the increase in carbon content remains as CO2 molecules in the water. Since the carbon content from bicarbonate and carbonate (KH) hasn't changed (only a small portion is converted to carbonic acid), the total content of carbon in the water increases due to increased CO2 molecules in the water.
In this scenario, simple agitation of the waters surface will blow off the excess (in the form of CO2) carbon content, whereas in scenario I above, surface agitation won't change the carbon related content, because that carbon content is almost entirely due to ions in the water, not gas.
So............................as you rightly point out, using the chart to determine CO2 levels in a non injected tank is pretty much pointless. I just have this bad habit of throwing chemistry at everything.