That can't be right. I have never heard of a pH probe going in a Ca reactor. But then again I have not heard everything yet.
The probe should prob. be monitoring the tanks pH. You could then occasionally check the pH of the reactors effluent. Also the addition of CO2 to the reactor causes the pH to drop to 7.0 or less so that the media breaks down into the base componets. Ca, Mg, HCO3, etc.
I have not used or seen a GEO reactor up close so someone else is going to have to help you.