You use a voltmeter to measure for stray volts.
Stray volts won't hurt your fish. Current is what can. And a grounding probe takes any volts in the water, creates current across your tank and out via the grounding probe.
So you are actually creating current where there is none, when you use a grounding probe, potentially a problem, especially if a device fails on you. Under normal operation pumps and heaters put voltage into your tank, mine runs 4-5 volts without a grounding probe.
The major benefit is that it could save your life, if for example your lighting falls in your tank when your hands are in it.
I have read about benefits for long term fish health, some stuff about lateral line disease and such, but nothing concrete IMO.
Do I own a grounding probe? Yes. Do I have it plugged in? No. But that is mostly from lazyness.
