Are my heater broke / overheating?

Jellybean123 · Jun 29, 2026

Hi, looking for some advice.

For context the UK has just had a big heatwave but today is much cooler. Last week temperatures in the UK hit 32°. My fish tank temperature was normal. I keep them at about 25°. Until one morning I came down to find some of the guppies dead. When I checked the tank, the temperature was 30° and my second tank temperature is 29°. I presume that this was due to the hot weather so I switched the heaters off over the weekend . Both tanks were fine for days whilst it’s been hot . Today it was a bit cooler. The tanks have dropped you around 22°. So I plugged the heaters back in. Only within a few hours the temperatures in both tanks have gone back to 30°. Both heaters still showed they were heating though despite being set at 25° temperature. The room temperature is showing at about 23° . I have now checked both tanks with multiple different temperature probes thermometers and they’re showing it above 30°. Is it possible that both heaters have somehow got damaged in the heat?

Do they just need to calibrate the tanks in their own time? What do I do with the fish? In the meantime? I have currently removed the heaters from both tanks

Uberhoust · Jun 29, 2026

Typically, you would only get one heater failing at a time. If they are an electronically controlled heater you might need to reset the heaters thermostat. If they are a typical heater using a bimetallic strip they should work ok after being turned. There is some adjustment that takes place as the water temp becomes the same throughout the tank but should not go to 30 degrees. One possibility is that there is some moisture in the heater and when turned off for a significant time the contacts have fused and may never open again. Just a thought.

Fishmanic · Jun 29, 2026

unplug and remove heaters...let the tank cool down a bit...then restart heaters but set them 5 degrees below present indicated temperature...if it still goes to 30 or above while room temperature is say 25 or less, then heaters probably are staying on and it would then probably be best to replace them.

Essjay · Jun 29, 2026

You could try testing them in a bucketful of cold water to see if they heat that to 30C

GaryE · Jun 29, 2026

Most incidents of heaters staying on and killing my fish have been after an environmental heatwave, just as you experienced. As soon as it gets above 25, I unplug every heater now.

I don't know the technical aspects, but over 60 years and many different tech possibilities, the pattern has been consistent. I trust heaters about as much as I trust salesmen. But I would unplug salesmen too....

bgarthe · Jun 29, 2026

I have found that my Finnex heaters will simply turn off/not turn on with warmer water. When my fish room gets to 77-78 a few times, my heaters just sit there. Water warmer than the set temp should actually prevent the thermostat from turning on as it’s designed to turn on with water temps below the set temp. Two heaters simultaneously going bad is statistically not very high.

Also, you can use a controller as I do. My BlueLine controllers are a back up plan in that they are set to turn off power to the heaters should the water temp get above my selected temp. My dual heaters in tanks are set to 77-78 degrees. My controller is set to 82. So…..as long as my water temp stays below 82, the heaters have power. The moment the tank water should hit 82, all power to the heaters is cut. Interestingly, my extra heater (set to 74) is powered, but never on until such a time that my tank water should slip to 74 if my two main heaters were to fail. This is important because my “stand alone” backup heater is sort of experiencing an ongoing “heatwave” as my tanks are kept above the 74setting and this extra heater is just on but never heating because my other two maintain the 77 degrees.