No carbon has nothing to do with ur ph. There something wrong with your unit the Ph of pure water is neutral 7ish this is because when your ro di works correctly it strips all the ions from the water leaving only pure water behind so you have a problem probably in your ro mimbrain
Ok. Checked the probe with calibration fluid and it is off 0.6. I checked it when I reinstalled my Reefkeeper and it was right. After retesting everything the good news is my RODI water is 8. Tap water is about 8.5. Tank is a little high about 8.3.
Bad news is my probe has changed that much in just a few weeks. I will recalibrate it and see if it holds.