Where the pH probe is located depends on exactly what you are using the pH probe for;

• safety for low pH on the system, or
• minimum pH for the calcium reactor effluent.

The more logical use for a pH controller and solenoid on a calcium reactor is the first, as a safety to ensure that the system pH does not drop to a low level if something goes wrong. For this situation, the pH probe is best located in the main tank or where water from the main tank enters the sump. It needs to be located away from where the calcium reactor effluent returns to the system, so that the actual current equilbrium pH of the system is measured.

In this case, the majority of the time the controller does nothing other than keep the solenoid open and keep the CO₂ flowing. But this should not be relied on to keep pH levels high enough on a daily case (such as overnight when pH levels naturally fall). If the pH of the system is falling low everynight, i.e. below 7.9-7.8, then there are some gas exchange issues that should be addressed. Shutting the calcium reactor off is not fixing the underlying problem.

The second use, to control the pH of the calcium reactor, the pH probe should be located in a small container. Into this container runs the effluent from the calcium reactor, which then overflows into the sump.

This is not a very good way to control a calcium reactor. The effluent pH should be controlled by adjusting the gas flowrate using the needle valve. Relying on a solenoid to do so can mean conditions fluctuate more significantly within the calcium reactor, making it more difficult to tune correctly. It can be used as a safety, much in the same manner as the first, but that can be problematic.

Note: you don't actually require a pH controller and solenoid for the normal operation of a calcium reactor (Do I need a solenoid and pH controller with a calcium reactor?).

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