My understanding is that the ppm numbers are the amount of CO2 required to drop the pH to a given level. KH resists changes in pH, so you need more CO2 to drop the pH to the same value when you have high KH. In contrast it takes very little CO2 to drop the pH in low-to-zero KH water (hence why RO water often measures quite acidic as it has absorbed a small amount of CO2 from the air).
At any KH, the optimum CO2 level is about 15-30 ppm, according to the chart. The idea is that 30ppm of CO2 will drop the pH all the way down to 6.3 if you have 2KH, but will only drop to ph7.2 if you have very hard (high KH) water.
But wait, now I'm confused! Are the yellow and red sections not the wrong way round in that chart???? How can 2ppm CO2 be "too much CO2" and 223ppm be "not enough CO2"???
Ok, give up. Let's wait for an expert to come along....
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