FWIW, i do use a grounding probe - for *my* safety, not the fishes. but its in the sump. the only electrical devices in the tank are near the outlets returning water to the tank, so if there is any current, the path is only a few inches from the powerheads to the outlet (and then down the plumbing into the sump....but since there shouldnt be any fish in the plumbing, the fish would only be exposed to current when swimming in those few inches between the powerhead and the outlet...which is to say, pretty rarely)

also, if youre not using a gfci outlet, i suggest you use a gfci pigtail, or retrofit a gfci plug onto your power strip (it aint rocket surgery cut off the old plug, wire on the new one and make sure you can keep white, black, green.....and water all separate!) this isnt for your problem specifically, just a good practice
heres what im talking about:

a not on GFCI's (real quick, cuz im wandering way off topic, but since we're talking about electricity i thought id throw this one out there)
identify items which you may *not* want on a GFCI, such as your return pump and skimmer
heres why: once tripped, a gfci wont come back on till you reset it. say you have everything tied into it and you have a slight power bump while away at work- something that doesnt bother anything else in your house but trips the gfci, youve potentially left your tank without any aeration for the entire work day. i had something *similar* happen to me once, i had an entire tank load of discus suffocate as a result. i wouldnt wish any experience like that on anyone
even while doing electrical work around the house i end up having to go around resetting all the tanks. GFCIs are sensitive!