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?