Aqueon Preset Aquarium Heaters require no adjusting! The preset temperature has been calibrated to 78°, appropriate for most tropical fish. This 50W unit, ideal for aquariums up to 20G,  is fully submersible, constructed of shatter-resistant quartz glass and features an automatic safety shut-off to avoid overheating. The heaters include suction cups and can be placed vertically or horizontally within the aquarium.  An LED indicator light displays red when actively heating.