Sudden Ph drop, what am I missing?

[SMB]

I did look up previous posts on this topic but I need further help.
I have a JBJ AIO 45 gal cube. It has been running without issues for 18 months.
Ph is measured by a Milwaukee ph probe.
Over 12 hours the Ph in my tank went from 8.2 to 7.1. This coincided with my weekly 10gal water change (actually occurred 48 hrs. post WC.)
I have never previously recorded a Ph outside of 8.2 to 8.4.
My dKh has not changed and remains at 9.5-9.8 where it has been for months. Ca++ also is steady at 450.
So I made a small amount of new Sw from my R/O unit and usual salt mix, to check it. Ph was 8.1
Then I took some tank water and put it in a glass jar and put the probe in it, Ph was 8.1!
That would explain why the tank occupants look fine.

The probe has been calibrated regularly but can there be some electrical issue causing the false reading? A crack in the probe or a short in one of three pumps/ two heaters in the back sump.
I tried turning off each electrical "appliance" one by one to see if the Ph reading would change but I could not seem to isolate the issue.
I am on my way to get a new probe, but appreciate any input. What am I missing?

 

[chipmunkofdoom2]

I would call this testing error. Perhaps something is sporadically leaking current into the tank and it just happened to leak when you were checking the pH. If your data are accurate and your pH is almost never outside of 8.2-8.4, and the pH is now reading correctly AND matching that of newly mixed saltwater, I'd put my money on testing error. Does the Milwaukee test and record pH values? Or did you simply look at the pH and notice it was low? It's possible that when you were unplugging and testing equipment that you caused whatever was leaking current to stop temporarily.

Another thing to consider.. the nitrogen cycle (the transformation of ammonia to nitrates by beneficial bacteria) can have an overall lowering effect on the pH. Usually this is not the cause of any issues outside of cycling, but I suppose an organism dying in the tank and producing an acute ammonia spike could contribute to a pH drop. I would think, however, that an ammonia spike large enough to cause that much of a drop would mean that something pretty big would have had to have died. Still, just some food for thought.

 

[SMB]

Thanks m2.
There is really nothing large in the tank to go unnoticed and all critters are present and accounted for. The fish are nano fish: a mated pair of Dartfish, Possum Wrasse, Citron Goby, Tailspot Blenny and a Pygmy Hawkfish, Perchlet.
I do have a Cowry that if it died might cause some issues but he has been chugging along for a year and a half.

The short answer is that I just added a new Ph probe from the LFS and the Ph is back to 8.0 immediately (and I have to calibrate it). All equipment fails at some point, and the old probe was the issue, I guess. I still can't explain the normal readings on water remote from the tank.
I am glad that it wasn't something organic.