Which do I trust?

[CanDo]

79 or 82 ?

 

[HuduVudu]

In tank thermometer.

 

[fishmonkey]

I have two eheims that go off around 81 but have it hooked onto an aquarium heater controller. The electronics used in these things are cheap and break down after turning on and off a few thousand times. By having an aquarium controller it goes through that and if it breaks you have a secondary check so you don’t boil your fish.

 

[Idech]

It looks like 28,6 C to me (83,5F) on the thermometer. I might be wrong, the picture is very blurry. You should trust the thermometer, not the heater.

Use another thermometer to compare, those can be off too.

 

[gbroadbridge]

79 or 82 ?

The thermometer