whats the temerature in your house? atmospheric temerature has a bigge effect on the temperature of your tank. one year my mom decided since we were outside the whole day she would turn the AC
off to save power, it was 30+*C and let me tell you, the temperature in my 26 gallon at the time launched up to 84*Fand kept climbing till i found out the AC
was on and thats why the house was gettign so hot.
some aquarium heaters don't have thermostats, atleast not accurate ones and they are set only to be adjusted to maintain a certain temperature above the room temperature. i no longer use that heater anymore and my tank has since been moved into my basement where the temperature is more concistant