I personally do not want to ever be shocked by my tank because it may only take 1 time for it to be the last. The grounding probe should be under water and plugged in that is all. You will need to check each of your pieces of equipment. I would start with the heater as they can be notorious for throwing out stray voltage as they get older and are possibly getting close to failing. Unplug it and stick your hand in and see if you still get bit. Of course it would be safer to use a voltmeter if you had one.
Also, do you get shocked in the display tank, sump, or both? If only one then that may help you narrow it down and you may actually need 2 probes.
I have also read that a small amount of stray current is normal at times and that it may actually benefit some corals. I know there was at least 1 person experimenting with adding voltage to the water column to test growth results. I do not know how that experiment ended or even if it ended. In the long run I feel it is much better to be safe than sorry, or possibly dead.
Best of luck getting it sorted out.