Aquarium heaters not only stop working during a power outage, but they can also break down. Most heaters will have a light that illuminates when they emit heat, and this is the easiest way to tell if it is working. You can use the back of your hand against the heater to see if it is slightly warmer than the water around it. If the aquarium temperature has dropped significantly without outside influence, it is likely that your heater is not working and will need replacing. Before replacing your heater, always check the fuse has not blown first as this is a cheaper and easier fix.