Attach one lead of your meter to a grounding rod in your tank and another to you ground connection for electricity (the third prong)
You will get some induced voltage from any motor in saltwater, its not easy to shield this all and no pump manufacturer to my knowledge sheiths their cables in braid or their motors in copper braid,
so as the ac current expands and contracts its magnetic field, the field causes a voltage to be generated in any conductive compounds or semi conductors, this is how generators and motors work.
Saltwater has the ability to work as a sort of battery and hold a charge generated by these fields.
If you test the ocean i bet you would get a small voltage as well,
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