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Adding CO2 into tank water will always lower the pH.
This is because carbon dioxide dissolves in water in the following equilibrium:
CO2 + H2O <-> H2CO3.
Thus carbonic acid is formed (the amount of 0.05 % or CO2).

Plants do use up the CO2 during day time and in even better in high light conditions, thus also carbonic acid levels drop and the pH is raised.

You should compensate this by increasing the KH (alkalinity, bicarbonate concentration) of your water. Generally when CO2 fertilization is ised, the KH should be about 4.
So do dose 1 teaspoon of baking soda (HCO3) per 50 gallons of water per day until KH is 4.

This will buffer the water against the pH drop.

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