I don't know what the effect is in freshwater, but I can assure you that
it is a very real problem in saltwater, and easily measurable. The probe
does sound like the opposite of what would be needed, but the trick is
in locating the probe near the object that's introducing the voltage.
That way, although a more coherent electric current is formed, it is
also kept away from the rest of the tank.
A simple experiment would be to put your hand into a tank when you have
a small cut on it. The cut will allow you to feel any stray voltage
quite easily. Then, install the grounding probe and repeat. This time,
the cut won't get zapped unless you put you hand very near the probe.
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