I had a close call with my wife discus tank. We usually keep the tank at 82 degrees. When I went to the tank to feed them I noticed that the water felt warmer than usuall so I checked the thermometer and it read 86. I then took out the heater to check it and it was set on 82 but was malfunctioning. I had to make a quick run to petsmart to get a new heater and am now in process of slowly working the temp back to 82. The discus seem to be fine but she lost 4 of her cardinal tetras. The heater was a visitherm stealth heater and I thought that those were pretty good.