Heater Question

[RIDavid]

I have a 300w Fluval heater in my 55 gallon Mbuna tank. When I wake up in the morning it is flashing red and saying it is 82.5 degrees. I have it set at 79.It has been getting very cold at night plus the lights are off. Why does the temp spike?

 

[This Old Spouse]

I have a 300w Fluval heater in my 55 gallon Mbuna tank. When I wake up in the morning it is flashing red and saying it is 82.5 degrees. I have it set at 79.It has been getting very cold at night plus the lights are off. Why does the temp spike?

I have yet to figure out heaters to my satisfaction. It could be because the lid is closed. Or because there's a malfunction. Maybe someone else has a good answer.