Just got zapped hard!

[hillbillyreefer]

Get a cheap multi meter, stick the black end into the earth pole on a three prong plug, put the other end in the tank. Get a helper to unplug things one at a time, when the voltage drops to zero the last item unplugged is the culprit. Good luck.

Edit:
When a heater went bad in my display I had the same shocking experience, I didn't notice any damage to the livestock in the tank. The fish were a bit skittish, but no harm done. The fish that were in there are still in there after about 2 years.