the grounding probe removes stray voltage from aquarium water.
SW conducts the voltage very well...basically it takes the electricity out of the water to the power outlet, on his ground line. If your power outlet is grounded (supposed to be but some old houses don't have some), you should definitely try it.
Considering your filtration + new fishies in the tank, the major fish stress shouldn't be the stray voltage, but still it should be resolved eventually.