Nathura works on a mechanical thermostat, are fully submersible and have an adjustable temperature.
The Nathura heater is glass, so needs to be underwater when turned on. If the heater is uncover when it is on (such as during a water change), then there is a good chance that the heater will crack. Turn off the heater about 10 minutes before a water change to prevent this from happening. If you have large fish, then these fish can knock the heater about, also causing it to crack. Heater guards are available.