The heat could be from the filter?
What is the heater set at.
It would not seem calibrated if your tank is warmer then desired or heater is set for.
I had a couple of those heaters and THEY SUCK IMO
I am Aqueon Pro for my 20+ tanks that need heaters...
If unplugging works then the heater is not accurate???
To me glass in the aquarium is bad enough, add to that a heater that is not correct ,and that completes my definition of junk.Junk I wasted my money on..
Some really say they like that heater? I sure never found out why.Mine are in the junk pile in case of an emergency...
My Aqueon pros have a lifetime replacement on them....