I turned off my pump and Co2 injection (I have a freshwater planted tank) as I was going to do my weekly water change. I have it programmed that if the ph drops below 6.21, the CO2 turns off. During water changes, I turn it off, because I have the tank open. The ph then usually increases to about 6.40 during this time period. I do have a sump setup, so the ph probe is in the sump area.

After about an hour, while I was cleaning the tank, I started getting text messages from Apex stating that my PH was low. I got 4 messages. I checked fusion and it said that the ph was at 6.18. When I was done cleaning the tank, I turned on the pump, turned on the Co2 and the PH immediately jumped to 6.38.

This is a lab grade Bulk Reef Supply Probe that is about a month old so it shouldn't need calibration. Is it a probe problem or a fusion problem. All software is up to date. Any idea what is going on?