that is a algea light on the sump and its 2 24" marine glow bulbs but i am going to change them to plant grow bulbs. the chiller has a thermostat that i have set at 78. and it also has a plugin for a heater, if the temp goes to 79the chiller comes on, if the temp drops to 77 the heater comes on. my tank stays between 78 and 79.