CO2 reacts with water to form carbonic acid.
CO2 + H20 --> H+ + HC03-
Therefore, you see a pH drop.
To determine the CO2 in the water, you need to know the KH(carbonate hardness). Then you can use this chart from The Krib or this table from Aqua Botanic to determine the CO2 concentration.
For pH 6.5, you will probably need about 3 to 3.5 dKH for enought CO2 for your plants.
BC
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