KH/CO2/Total Carbon is a complicated ball of wax depending on what you
are specifically talking about and which environment it is in.
The theory is fairly well dealt with in chemistry class.
But the practical matter and applying it to plant tanks is another
matter.
Test the KH, go to the pH/KH/CO2 table and follow it down till you
find the pH you need to have a CO2 level between 20-30ppm. Add enough
CO2 gas ONLY(no acids/"buffers" etc) to get this pH.