I do my stray voltage testing at the 10 setting, and if the needle moves at all, I know I have a problem. Then it is just a process of elimination to find the culprit device.
I have found a couple mag pumps that moved the needle slightly. But the worst was an old T8 light fixture that was on top of a QT tank, and somehow the voltage would get down into the water. I am not using that one any longer.
I now test all my tanks, and sumps, and use the Rid Volt grounding plugs.
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