Out of curiosity can the pH proven be thrown off by a magnet? The reason I ask is I put a new apex in one of my tanks, calibrated etc. Probe was reading around the 7.8 for a few hours. Decided to put in an optical sensor for a low level warning and I believe it “clung” to the probe. Possible that it was instead clinging to the probe holder. I noticed when this happened that the reading dropped to around 6.7-6.8 and did not recover. I recalibrated it and it was back at 7.77- 7.8. Tonight I shut the skimmer off to empty and clean the cup and noticed when I shut it off it dropped down again to that 6.8 range the second the skimmer shut off. The probe is located in a the return chamber though.