On Friday I installed a new submersible heater (the old one was recalled) in my 10-gallon tank which was filled with fancy-tail guppies. The next morning almost half of the fish were dead. I removed the heater and 12 hours later the rest were gone as well. I have had the tank in use for at least 12 years and have never had a problem. The water temperature was fine, about 78 degrees. A friend suggested that the heater may have had a short circuit and my fish were electrocuted. Any thoughts?