something interesting i just learned, its better to slowly give your tank a swing in temp till it gets used to it, and always have a swing, of 2-3 degrees. your corals become much more tolerent in the event of something going wrong.
when your corals are completely used to a temp and never fluctuates, any little spike causes them to go downhill because they havent built up a tolerence to this. corals in the wild see huge ranges in temp throughout the day.
i keep mine exactly at 81, but am going to let it fluctuate from 79-82