When I have mine set to 78 it only goes to 80 durring the 8hours my lights are on.
When I have mine set to 80, it jumps to 85 when the lights are on. I cant figure out why one is a 2 degree diff, and the other is a 5 degree diff, so i keep mine at 78-80. When I can afford a chiller, Ill keep mine at 82.
Corals do best in warmer water than fish. 80-82 is generally accepted as best. But don't go past 84.