I had a tank full of bettas I was treating for ich. I turned the heater (Eheim jager) up to 30 degrees Celsius and put a towel over their tank as the medication is light sensitive.
I noticed that the tank felt quite warm to touch and that my bettas were looking very lethargic when I lifted off the towel.
Put one of my thermometers in, and the temperature was a whopping 35 degrees Celsius, or 95 degrees Fahrenheit. I turned the heater back down to 24 degrees, took the towel off and the lid and thankfully it has already dropped back down to 30 degrees. My fish are now out and about and have just had a big meal of white worms.
It just reminded me how important a thermometer is when using heaters. It was only that it seemed so much hotter than my other tanks that I thought I would check on it. Luckily everyone seems okay and I think I cooked the ich haha