I forgot to refill my ATO and I kept meaning to and the water level dropped so much that the heater was mostly exposed to air. And as soon as I added more water I heard pop and fizz! And smelled burning. Oh bleep! Unplug everything immediately and see my crispy heater but very safe fish and no electric shock or fire. The lesson here refill the ATO as soon as it runs out. The heater lasted me a solid two and a half years when I have forgotten to refill the ATO many times and refilled it the next day or two. I think this time I just waited way too long and the heater ran dry. Thank God I noticed it when I did because any longer and I know a fire or an electrical shock would have happened.