Tricky question, certainly wouldn't want to wrap a copper wire around it, that would harm the fish, I would say based on my experience in electronics, if it has a ground plug it should be grounded out to a dead metal part(the case in this instance) which should in theory pull out any stray voltages in your tank, one way to test is to measure stray voltage from your tank with a multi meter, then just connect your multi meter between your ground plug and ground to see if you're getting any reading and if its about the same as probe in water to ground. Of course that's if you even have any stray voltage in which case your best bet is to find the offending device and replace it.
