At this time of year, it is easy to a tank temperature to be high. I assume you cannot see if the heater is turning on or off since it is in a back chamber, so you can try unplugging it for a day and seeing if the temperature drops. If it doesn't, then I would say that your room temperature is the issue. I don't use heaters in my tanks at this time of year, for this reason. It generally isn't necessary, unless you are really cranking the AC.
If the temperature does drop, then the heater may not be working correctly or being in the back chamber does not allow for enough circulation, creating an issue with properly heating the tank.
Date published: 2016-06-28