I use the American Marine Pinpoint pH Controller. The solenoid is controlled by one of the outlets in response to low CO2 levels (that is, high pH) and I have an airpump/stone on the other outlet that is activated in response to low pH. The control range is within ~0.10 pH units and it is easy to calibrate and otherwise maintain.
I have the same one. Thing is, my pH is already kinda low at 6.01 to 6.50 so my co2 rarely kicks on because I don't think it would be good to be any lower. It kind of defeats the purpose of having the automatic co2 system if the pH isn't going to be high enough for the the co2 to kick on. Is there a solution to my problem?