The way a ph controller is usually used on a calcium reactor is to control the ph of the effluent. An easy way is to stick the probe in a cup or tube and drip your effluent into that, letting it overflow into the sump. Some calcium reactors have a fancy port for a ph probe to go right in the reactor, but it's not necessary and honestly I never looked into it in the reactors I've made because it presents a possible leak point, and I really hate leaky equipment.

Anyhow, it works by activating a solenoid on your CO2 regulator, so that when the effluent ph drops below a certain level, it shuts off CO2 flow to the reactor. Then eventually the effluent ph rises, which turns on the solenoid and CO2 flows to the reactor again. It is a nice feature, but if you have a good way of controlling CO2 flow like a dwyer flowmeter, and a reliable way of controlling the reactor effluent drip rate, then you don't need one.

I've built lots of reactors and helped lots of people use them. IMO, the trick is to get good control of the CO2 flow and effluent rate. None of the typical regulator needle valves that I've seen work even close to as well as the dwyer flowmeter, i think it's model RMA151, but it's been a while so I'm not sure. It's the one with a flow rate of 5-100 CC/min if I remember, and it's about $40 from dwyer instruments.

For the effluent, I've had the best luck by far with a mini ball valve feeding the reactor off a tee on the return line, and a nice pinch valve like they use on medical drip bags on the output. In my experience, it requires some fiddling no matter what you do.