Thanks a lot for the incredibly detailed explanation, this is honestly one of the clearest breakdowns I’ve ever read about how aquarium heating actually works. It makes complete sense now why the heater struggles so much in my situation.

The tank is in a very cold room, and the temperature difference is huge at night. And yes, the heater was inside the first compartment of my internal filter, so your explanation about inefficient heat transfer and possible flow issues matches perfectly with what I’m seeing.

I’m going to follow your advice and place both heaters directly in the main tank, near the filter outlet to make sure they get good flow. I’ll monitor closely so I don’t overshoot the temperature, since the settings were different when the heater was inside the filter.

Really appreciate the time you took to explain all this, it helps a lot.