We need critical review on the accuracy of the video claim that CO2 can eat kH. I am skeptical of the claim too, as many of you have not experienced kH drop using CO2.
That said, depleting kH does not necessarily make water soft, unless it also depletes Ca and Mg. So if the claim were true, injecting CO2 is not a cheap alternative to RO water.
I converted by my cichlid tank to planted cichlid tank, and inherited dolomite substrate. Additionally, I have a few Texas Holey limestone rock to bring up the hardness. So my experience with CO2 is different from most you who use other substrates. Dolomite is calcium magnesium carbonate, or CaMg(CO3)2. Texas Holey rock is largely calcium carbonate CaCO3. I am wondering why not more aquatic gardeners use Ca/Mg based substrate rather than to dose gH. Gardeners regularly apply dolomite limestone granules to sweeten soil and make lawn green.