PH probe giving false low PH reading

[RyanScanner]

I just want to see if this has happened to anyone else.

I have a PH controller and probe in one of my planted tanks. Now this PH Controller is saying my PH is suddenly 5.4, it was 6.8 this morning 12 hours ago. I recalibrated with the solutions and tested it in another tank. The results matched pretty close to that controllers reading of 7.2.
I then used two different liquid test kits they one said 6.4 the other 6.8, one is old so no surprise there.
Controller still say 5.4 even though in other tank it matches.
I then tested a bowl of water from that tank and outside the tank it tests at 6.8!
I then did an 80% water change with my treated tap water that is always 7.0 without fail. Chemical test says 7.0 PH Controller and probe 5.4. Wtf!!!

What could possibly cased this. Something in my tank is throwing off the probe. The probe functions 100% fine outside the tank even with the tanks own water. I’m dumbfounded.

 

[Milingu]

Suspect number one is likely to be electrical voltage from another device that influences the measurement of the pH probe. A very small voltage is enough to distort the measured values, so sometimes it is even enough to simply measure at another point in the aquarium.

If that doesn't help:
Unplug all devices including the lighting on the affected aquarium, and then measure the pH again. Another option would be to ground the aquarium.

And yes, you are not the only one with this problem.

 

[RyanScanner]

I have a grounding probe in that tank but that is a good point. I’ll isolate each device and see if anything makes a difference.