Aquarium heater

[1] Most tropical freshwater and marine aquariums are maintained at temperatures that range from 22 to 30 °C (72 to 86 °F).

[1][2][3] The types include glass immersion heaters and undergravel heating.

Some glass immersion heaters also contain sand, and most, but not all, are fully submersible.

[1] The glass tube also contains an adjustable thermostat which turns the heating element on to maintain the required temperature.

[1] Undergravel cable heating is popular in heavily planted aquariums.