No, without getting into the chemistry too much, CO2 does not lower KH. Because H2CO3 (carbonic acid), is an acid, it is assumed that it will lower the KH. This is not true as only the chemical species change which provide alkalinity.
H2CO3 --> H+ + HCO3-
HCO3- --> H+ + CO3--
The pH will change, but the KH remains the same. The net change is zero.
KH drops when a acid with a pKa (dissolution constant) greater than carbonic acid is added to the water from fish metabolites, bacteria, decay, etc. This lowers the KH as it exhausts the ability of the primary 'buffers', e.g. carbonate, bicarbonate to accept any more protons....(H+), called the carbonic acid equivalence point.
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