Can kill YOU, not the fish. Electricity can defibrillate your heart above 100mA for the typical person but has been recorded to damage pacemakers as low as 30mA. Getting shocked when you are covered in saltwater can reduce your body's internal resistance to as low as 1000 ohms. This would allow 100mA to be passed through your heart at as little as 100V.Just wondering, how would that kill your fish?
This is a worst case event and something as simple as wearing shoes is enough to save you from death here, but if everything lines up wrong where you are barefoot, covered in saltwater and reach into a tank with a fully failed heater, you're a goner.