Weird temperature changes

[Kirsty]

I have a stick on one and one that sits in the water.

 

[colegamby]

what kind of light do you have. Incandescent lights give off alot of heat and can heat up your tank if they are on for awhile.

 

[Slyspy]

I had that when I used a heater that was too powerful for the size of the tank. By the time the thermostat registered the temperature rise the heater was going full whack and continued heating the water to a much higher temperature due to residue heat. I replaced it with a less powerful but otherwise identical heater and had no such problems.

 

[Kirsty]

Well my heater came with my tank, so i am guessing it is of the right power.
I think one of the reasons is that i had stuff sitting on the tank lid....