I also use grounding probes, my fish weren't dying, but I would get shocked when the heaters crapped out when doing a water change or scraping algae.
A simple test, is to stand on the floor barefoot and dip a finger with a small cut or scratch on it in the tank.
I would not do this unless totally healthy, and there is a proper way to do it so the shock does not pass thru your heart. I believe I saw the technique in an AWWA publication, but I no longer have it.
After doing the test I start unplugging things and repeat until the shock is gone. Could be anything, a heater, a pump, even lights.
I would not recommend the test though if even a slight heart glitch.
