Setting the CO2 to deliver a consistent 20-30ppm of CO2 for the volume of water is just so much simpler of a way of going about this. It isn't effected by any other variables than the volume of water in the tank.
The use of a controller simply introduces far too many possible points of failure. If the water supply KH changes, suddenly the controller will inject the wrong amount of CO2. If there are other sources of acids in the tank (driftwood for example), the controller will dose the wrong amount of CO2 because the pH will fluctuate with every water change. If the probe fails or isn't calibrated regularly enough, again, wrong amount of CO2. On and on...
Why on earth people pay for controllers is beyond me, that element of safety is really just introducing several unnecessary risks.
So, yes, there are so many possible variables that will effect pH as to make using a controller a foolhardy endeavor in my opinion.
The KH/pH/CO2 relationship only works when CO2 is the only acid and you know all of your buffering agents are covered in the KH test. This is why we use drop checkers with calibrated solutions; testing in the tank has too many unknown variables.
What folks in the past, and sadly even today apparently, thought were pH changes effecting fish were really changes in GH, KH, TDS, the whole pH craziness is just that.
Clearly the OP is worried about the water quality for the Discus (as mentioned in another thread); they come from ultra soft, practically zero TDS water, adding buffer that will raise the pH, adding buffer that the fish will notice as a TDS change, that doesn't seem at all a wise choice. CO2 based pH changes, on the other hand, are an 'invisible' change for the fish, provided one hasn't turned the tank into a gas chamber of course.
The KH test could be bad, any test can be. I have a GH test that gives me fantasy land results every time. However; with the OP adding "RO Right"
we know there is KH being added, it says so on the label of the product. Has anyone here, other than me, read the description? It's a complete reconstitution product, meant to replicate the soft river water that is so ideal for these fish -- including alkalinity.
The emphasis that is continually being put on pH should be instead put on TDS.
In any case, it was not my intent to debate the topic, I was just offering my help in the form of practical advice.