You can measure the CO2 by using PH only, eliminating the effect of non carbonate alkalinity and acidity from other than carbonic acid. Just take a sample of the water in the tank, let it sit in the open for a few hours for the CO2 to dissipate down to the atmospheric level, then measure the PH of that water. To get 30-40 ppm of CO2 in the tank you need to decrease that PH by 1.0. You can use this as the controller setting, but it will only be accurate until more tannins get into the water, so you almost have to repeat this every day or so until the water in the tank is stable, then start all over when you change water. One caution: while a 1.0 drop means 30-40 ppm of CO2, a 1.3 drop means twice that much, and a 0.7 drop means half that much. This makes measuring PH accurately very important.