Saltwater especially is prone to this. The salt eats through things. Plus you have all those inductive devices which impart a charge to the water. Both fresh and salt will end up with a voltage compared to ground.
Easy fix - Coralife Ground Probe PLUS GFCI breaker/outlet. The breaker should trip, but in the event that it doesn't, the ground probe SHOULD provide a better current path for any stray voltage compared to the one you might otherwise be providing with your hands and feet. ( hopefully )
I've been able to see the difference in my fish's behavior when I ground the tank compared to when I don't. Literally watch them change as I plug/unplug the ground probe a few years back. So I always ground the tanks now, it just seems prudent insurance.
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