The grounding probe is probably what did you in. I am NOT suggesting that anyone do something different but if the tank isn't grounded then the electricity has no where to go and won't flow through your water. With the ground, you provided a path for the electricity which unfortunately spans the entire length of your water system. Without a ground it would be like a bird on a bare wire...they don't get fried because the electricity has no where to go.
Don't know aquariums as well as I know electricity so not sure if this is possible but if you can, consider where you put your ground. If you can ground the top of your water then the electricity will flow from pump or heater to the ground. Of course, this all goes to crap if something in your sump breaks and leaks electricity into the system. Maybe the better answer is to put multiple grounds in the tank so it'll go to the closest ground instead of through your corals. It's just so hard because the more salt the water has, the better of a conductor it becomes.
Does something like this happen enough where if I built a detector many people would have interest in it? Something that either shut down the power to the tank, or turns on a warning light, or send warn via some other means? Could simply have a beeper that goes off until you fix the problem. The electricity shouldn't bother the tank creatures too bad if you get to it quickly.