CO2 injection will certainly reduce pH as carbonic acid is formed.
The table in this link below shows the relationship between kH, pH and CO2 concentration.
pH/kH table
going by it, a pH of 6.4-6.7 would be alright. Maybe you can try positioning your filter outlet so that it agitates the water surface more and increases CO2 loss rate.
But if your fish/plant show no sign of distress (like gasping at the surface, esp in the early morning), I think there's no need to worry or tinker much with it.
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