A small submersible heater sized for betta tanks and nano aquariums up to 5 L, where a standard 25 W heater would cook the tank. The body is a durable polymer rather than a glass sleeve, which is the sensible choice in a small tank that gets moved and cleaned often. Fits to the glass with a suction cup and has a red indicator light.
How it holds temperature — read this part
The heater has no dial. It self-regulates against the volume of water around it, so the temperature it settles at depends on your tank size and room temperature: roughly 30 °C in a 2 L tank and around 26 °C in a 4 L tank at 22 °C room temperature. That is fine for a betta in a 4–5 L tank and too warm in a 2 L one. Run a thermometer with it and be prepared to size the tank, not the heater.
Pair it with a stick-on LCD thermometer, and see the rest of the Marina betta range.