I think you guys mean a pH controller, not a pH monitor. A pH monitor is going to do just that, monitor your pH. It gives you a reading on the current pH.
A pH controller works in conjunction with a solenoid, which is built into both the Milwaukee and the JBJ regulators. The controller has a switch built into it. You plug the solenoid from the regulator into it. The controller will then turn the solenoid on the regulator on and off.
You will set a pH on the controller, say 7.0. If your pH is higher than 7.0, the controller will power the solenoid, which will open up the solenoid valve and allow
CO2 to flow into your aquarium. The
CO2 will flow. Once the pH in your aquarium reaches your set point, 7.0 in our explanation, the controller will detect that the pH has reached the desired set point, cut the power to the solenoid, which will close the solenoid valve and stop the
CO2 from entering the aquarium. If the pH creeps up, the equipment repeats the process. if the pH remains stable, the process never starts. The whole thing becomes automatic. No need to worry about swings in the Discus' aquarium!
Mike