In fact the heater stopped working today. Eventually already yesterday. Since I held my hand onto the front shield of the tank by 'coincidence' I noticed that the water was quite cool. The thermometer showed 17'C/63'F which seems to be quite scary. Fortunately I have an older used heater that still seems to work. Additionally I use another undersized tank heater to accelerate the warming process. I really hope that the decrease did not and will not do any harm to my little friend. I thought about the fact that in 10 meter depth in the ocean there might be a similar temperature or even less than 17'C/63'F. How do you see this?