Like AquaAurora, I heard of horror stories with those betta bowl heaters. From what I understand, they heat the bowl 10 degrees above room temperature so if the room varies, so does the bowl. I personally want a heater that keeps the small tank at a consistent temperature.
The Jager 25 watt heaters are way too physically big.
I wish they still make the ittle Compact Themal 25 watt heaters because they were perfect. I'm still using two of those. But I did find the 25 watt Marina Mini Submersible Aquarium Heater which is small enough. It's been working for 7 or 8 months so far and is adjustable.
Those Catalina heaters mentioned by devilduck look good. I think I'll try one of those next time if I can find the 25 watt.
2 2.5s, 3 10s, 2 20s, 1 29. Low light, low tech. Ponds