Sounds like you've got a bad pump or heater. Like twilliard said, unplug things one at a time and recheck the reading. You might have a few things leaking.
I'd suggest you try and track down the source of the voltage in your tank too. It is normal to see a few volts. All pumps are going to induce some amount of voltage in our systems when they are running. I'm not sure what you mean by your comment about voltage vs current killing the fish. Voltage is a measure of the potential difference between two points, current is the flow of electrons between two points. Yes there needs to be a current (flow of energy) to kill a fish, but having the voltage potential means the only thing lacking is a path to ground. Maybe I don't understand what you were saying though.