Here is something from Ron Shimek.....
Question: The tank is reaching about 88 degrees in the apex of the lighting cycle. Last year ran about 82. It has fans on the sump as well as the top of the tank not to mention four computer fans in the canopy. I have done everything I can think of short of floating ice blocks in the sump. Animals don't seem any worse for wear so I am not exactly in a panic yet.
Answer: Given that they are just getting into the normal range for coral reefs (82-84 deg as average), I would suspect that instead of being in a panic, they are figuratively breathing a sigh of relief.
My tanks routinely will reach those temperatures in the summers. During two diving trips to Palau (richest coral diversity in the world....), one trip in April, one trip in September, the temperatures in the diving zone averaged about 89 degrees. Cool upwelling water measured 86 degrees, and hot water coming off the shallow lagoonal flats was about 95 deg.
Most natural reefs have massive temperature swings, though, on any scale you wish to measure, daily, monthly, annually, so that is not a big deal.
I would worry about prolonged (several days- couple of weeks) temperatures above 90 deg. One thing, if your system has been normally kept at 82-84, the organisms in it will be a lot more tolerant of the higher temperatures than if they have been kept in the cold (75-80).
Fluctuations are really not a problem - in some higher latitudinal reefs the fluctuations are enormous. There are data from Johnson Atoll, showing fluctuations on the order of 20 deg F in a single day (from about 72 deg to about 92 deg), and guess what... The reef's still alive...
What is more important are the extremes and the duration of the extreme values. Temperatures over 90 for any extended period is a problem, particularly for corals normally kept at cooler temperatures. I would watch the temperatures and attempt to control the environment so that temps over 90 are avoided.