An aquarium thermometer is a device used to measure the water temperature of an aquarium. It typically consists of a glass tube attached to a sensor that is submerged in the aquarium and registers the temperature of the water. It is important to use an aquarium thermometer to ensure the temperature of the aquarium is within an ideal range for the type of fish and other aquatic life that inhabit the tank. Accurate temperature readings are essential for the health and well-being of the fish and other aquatic life in the tank.