I believe what Greenbean is refering to is that keeping corals within a degree of variance in temperature fluctuation can make the coral more vulnerable to occasional fluctuations of temperature.
So don't try to keep the tank at 80F all the time. Difficult to do and likely to be bad for your corals in the long run. Plus, one hot or cold day and you could lose lots of corals.
In my experience, allowing your tank to get over 84F will cause death in some of the more sensitive species. (Yuma Ricordea).
I prefer to have some fluctuation between day and night (79F) and daytime (81F). That is a four degree change in a single day. Much more than that, and you will likely see more fatalities than trying to keep the tank at a single temp all the time would create.
According to Walt Smith (major propagator and farm operator) The water off of the islands of Fiji (where he farms his corals)stays in the mid 70's year round. If anything, his recomendation was that we keep our tanks too warm for most of the corals being brought in from the South Pacific. Although I understand this is frequently debated.
There will always be people saying our tanks should be warmer. From my personal experience, I see no harm keeping the tanks in the mid 70's, but do start seeing mortalities as soon as the temp gets above 82F.
