When that happens usually there is an exposed wire from a old heater or other electric item in the tank. Check your heater for cracks too as I believe that can cause the shocks.
Or maybe you are just conducting a lot of electricity yourself. Do you walk around on carpet with socks? or better yet... wool socks? lol. Good luck figuring it out. and be careful!!
Lol. Then maybe your socks are the culprit! I have found that if I wear wool socks around and then go poke my finger in the tank i get a bit of a shock similar to touching a metal door handle or car after walking around. Anyways, It may not be what you are experiencing, I just thought i would get a laugh out. good luck again.
I can't think of anything but a busted heater. I always wear socks, and almost my entire house, including my room, is fully carpeted. I'm surprised the fish are alive, lol