Hello Tom, I have that exact reactor and have done all the mods you've probably read about and I'll share my experiences over the past year. This is what you need to add a ph probe,
Korallin Lid with pH Probe Port for C-1502/3002 and this is something else that will make your life easier, a pinch valve,
Flow Control Pinch Valve
I did the needle valve mod but was unable to get my effluent dialed in. For example I'd have 30 drops per minute going and the next day it may change to 10 drops per minute. After I called to get tech support they told me the reactor may be building to much pressure and to open the effluent vent a little. Well this helped but with the pinch valve I can zero it in and it stays consistence. Also a good regulator is a must. Lot of people run them a lot of different successful ways. I have a friend that has a range set on his apex, something like 6.3-6.8, and uses the solenoid to control everything. Personally I bought a good regulator,
Supreme CO2 Regulator | Green Leaf Aquariums, and use it to control my reactor and the solenoid and apex ph probe as a safety backup. I know expensive regulator but I'm a numbers person and it was killing me to have to make daily adjustments with my reef fanatic regulator. Again I have friends that run the RF regulator with no issue.
Next everyone talks about the effluent dkh numbers being above 30. Well it took me forever and several forums to figure out how to measure my effluent dkh with a salifert test kit. This is what I came up with and it seems to be as accurate as any I've tried. With a Salifert kit use 1ml of effluent and 1 drop of KH-Ind and run test as normal but multiply your result by 4 and this if your effluent dkh. Example: After testing and color change I see the bottom of the plunger is at 0.36, this is 9.9 KH, so 9.9 X 4 = 39.6dkh.
Also I've read a lot about the ph inside the reactor. I use ARM course and found around a ph of 6.25 gives me the effluent dkh I'm looking for and keeps my DT Alk in range and corals look happy. I did read anything below 6ph can turn your media into mush but it seems really difficult to get far below 6.2. My system is a 180 gallon mixed and I'm dripping about 20ml of effluent a minute and my C02 bubbles per min around 33. This gives me a dkh of 39.6 and Alk in my DT at 8. All systems are different but this gives you an idea.
Something important also is to have your system numbers close to what you want them to be. This will help also get your reactor dialed in. Hope this helps and anything I can help you with I'll do my best. Remember there are a million ways to be successful
