From what I remember reading a while ago , and I may be completely wrong but here goes.
Stray voltage if too high in the water can affect the fish , but won't give you a shock if you put your hand in the water. Stray voltage usually comes from powerheads and other pumps that use magnets. I have about 25volts right now when I test with a multi meter and if I add a grounding probe it will drop to 0. I'm not using the grounding probe right now because from what I've read like gregzzz wrote earlier , is that using a grounding probe with no gfci is very dangerous. I'm in the process of installing a gfci and then I will start using my grounding probe.
Stray current doesn't bother the fish but this is what will give you a shock. Stray current comes from things like a faulty heater.
To test stray voltage with a multimeter set it to volts.
To test stray current set it to amps or milliamps.
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