The addition of CO2 does not lower KH to any significant degree, only the pH. So for any given KH, adding more or less CO2 will change the pH levels of the water. In theory, if you are accurately testing your KH and pH levels, you should know your CO2 concentration.
A KH of 10 is fine for growing most all plants. People add carbonates when they have very low KH or are using RO water.
In some locations, water utilities change their source water during the year. If this happens, your tap water KH can vary by quite a bit. So if you're adding enough CO2 to keep the pH at a certain level, when the KH changes (from water changes) your CO2 concentrations are no longer the same. So it's a good idea to measure the KH from time to time.
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