CO2 will work, but as Reeffish mentioned, this will only work if it is actually connected to a CO2 source. There are a few things that should be addressed before you proceed.
First, how did you calibrate your pH probe and does it read the calibration correctly after calibration? You should be using a new standard 7.0 and 10.0 calibration fluid. The probe should be cleaned and rinsed in RO/DI before and between calibration points. Also, I believe if the temperature of the calibration fluids is significantly different than the tank temperature, this will throw off the calibration, but I may be wrong about that sensitivity.
Second, how did you route the wires on the pH probe? The pH probe, ORP probe and conductivity probe are very sensitive to electromagnetic interference.
Third, how are you dosing your alkalinity? This will also impact your pH. If you are dosing kalkwasser or a alkalinity additive for example in a single daily dose, this will cause a drastic increase in pH. A calcium reactor, conversely, will cause pH to fall.
Finally and Personally, I don't think this is necessary or a good idea and you could be playing with fire as accidentally dropping your pH when something sticks too low will crash your tank, while leaving the pH on the high side will cause no harm. Also, note the pH probe is prone to errors, interference and cycling and should not be used as a control point for your tank (same for ORP and Conductivity). Its a good failsafe, check a kalkwasser dose (dose pump off at pH>8.5) or for a calcium reactor (CO2 off at pH < 7.8) for example, but should not be used to actually control it. It just is not that reliable and you will cause more damage.
Heed this word of caution from Randy Holmes-Farley:
Source:
http://reefkeeping.com/issues/2004-05/rhf/
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