UPS Battery backup stray voltage issue

[Ladder Man]

I installed a Pecron E2400LFP Portable Power Station 2400W 2048Wh UPS backup on my tank a week ago. After a few days, I started having wildly fluctuating readings via my Hydros for both PH and the KH Guardian Aim unit that also uses a PH port. I contacted Hydros and they had me put the probe in a cup of tank water which caused the readings to go back to normal. They advised this meant stray voltage, so I got out the multimeter and started unplugging things, but could not isolate the cause. It wasn't until I tried unplugging the Hydros and other power strip from the UPS and plugging them back into the wall that the problem went away. I do have a grounding probe on the tank, but this did not help. Any ideas what was going on here?

 

[Timfish]

It seems pretty obvious your power station is causing some kind of interference. It would be interesting to use an ocilloscope to see how noisy the output of your power station is but I realize that may not be practical. Even though it should be a true sign AC output it might be noisy. One thing I would do is check to be sure the ground on the output of your power station is in fact connected to the ground on the input side (it should be but things happen). If you do have a good ground on the output I would be thinking the AC output is noisy. Unfortunately, having a ground probe in your tank won't help much with a noisy AC output but you can try moving to next to your pH and KH probes.

(FWIW, I would only use a power station to run a return pump and maybe a heater, the less that's plugged into it the longer it will run in the event of an emergency.)