Hold on scape, 6.6 should be the pH of the drop checker. Any number of things could affect the pH of the water. If you are using a controller, you should be focused on the pH drop between CO2-on, and CO2-off. Reference the red and blue highlighted lines.
Very good points to add, everyone!
Oh okay btmmer92, I see 6.6 on the contoller and the drop checker is green... (using reference 4dkh water)