Why do people brag about keeping their temp, even to the point of advising others, to keep their tanks temp at 78-80 when The temperature of the water at Australias Great Barrier Reef will go from 75*f during the winter to 86*f in the summer?
Corals can survive in a pretty broad range of temperatures. Slow changes is the key as corals hate quick temperature fluctuations.
I think the 77-80 range is just a safe zone so you can swing a few degrees in each direction and still be at an acceptable temp. They can do fine outside of these temps as long as they are acclimated properly to it.