Underwater Treasures Floating Glass Thermometers are mercury free for fish safety. The thermometers measure 4.5 inches in length and include a green-band on the temperature line to indicate the correct aquarium water temperature for optimal fish comfort and health. The glass thermometer can be cleaned as needed and the smooth glass surface resists fogging and algae growth. Underwater Treasures Floating Glass Thermometers require no batteries or special equipment to operate and are guaranteed to be accurate.