This aquarium heater is preset to maintain water at 77 ℉. Although this factory temperature setting cannot be adjusted, it does meet the ideal temperature range for most aquatic environments. This heater is designed for both plastic and glass tanks, between 2 – 5 gallons in size, and is safe for freshwater and saltwater aquariums. Small, Safe, and Effective: This aquarium heater is equipped with an automatic safety shut-off feature that cuts power to the unit once it’s exposed to air. This unit provides excellent heat distribution over the entire element. Furthermore, the two included suction cups allow you to mount it horizontally or vertically in your tank. Please Note: Always ensure that the unit is at no risk of damage from falling or moving aquarium décor, is positioned in an area of high flow to ensure efficient heating, is always completely submerged in water, and is the right wattage for your aquarium. Ambient room conditions can affect the efficiency of any heater as well.