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Adding dissolved CO2 to the water will lower pH. I am wondering if the inverse is true, lowering pH by using some portion of RO for water changes will that allow more CO2 to be dissolved in the water?

Current parameters:
pH - 8.1
KH - 8

Using the chart shows ~2ppm dissolved CO2. Reducing pH to 7.5 (KH to 6 I assume would go down due to RO) increases dissolved CO2 to ~6 ppm a 3x increase. I realize the chart may be inaccurate in this particular situation (no CO2 injection).

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By adding CO2 to the tank you are not lowering the pH directly but rather making the water more acidic which in turn lower the pH. So to answer your question no it does not work the other way around.

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If you lower the pH by adding CO2, you will have more CO2 in the water. If you lower it by adding a bit of strong acid, the equation that ties CO2 concentration, KH, and pH, will no longer be true. So, the charts based on that equation will also be untrue.

Similarly, if you raise the pH by decreasing the amount of CO2 in the water, you will have less CO2 in the water. If you raise it by adding sodium bicarbonate to the water, will have a higher KH, but not less CO2.
 
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