Weird temperature increases

[RyanScanner]

I’ve got a 300g planted aquarium with a UV steriliser and 2x 500w dolphin heaters (brand new) I replaced the old ones.

I keep getting weird temperature increases from 25c to 28c. The room temperature is not above 25c during the day. I turned off the UV and replaced both heaters in case they were faulty. It’s still happening.

Ive got 3 thermometers 2x probe and a mercury one all within 0.6c. I’ve just turned 1 500w off See if that helps. Just can’t think why it’s happening.
It was stable for the past 10 months.

 

[HarleyK]

Maybe your heaters have a larger temperature range where they kick in and where they turn off again. Unless you have a central temperature controller. Just a thought.

 

[RyanScanner]

I have a central temperature controller and the heaters are set to a max 26c. I suspect my light is heating the water. It’s an enclosed tank because I have eels. So the light is under the lids for aesthetic and efficiency reasons. I’ve turned off one 500w heater and I’m managing to keep it at around 27c.
I’m looking for a better quality light with a better heat sink.

if the heat sink is built in is it thermally insulated? Or will it still be the same due to it being enclosed so no where for the heat to escape?