My sps reef stays between 82-84, and I have never had a coral bleach.
The following info is taken from a member of another large board who is a marine biologist. Again, not my words, but this is verbatim in which he wrote me back.
I first asked him about how much the ocean temp can fluctuate throughout the day. His response:
"I guess I'm not sure exactly what you'd like to know but I'll try to answer. Over the course of 24 hrs the temp. can be very stable on some portions of a reef and very unstable on other portions. On reef flats, in shallow lagoons, and in areas without fast rates of exchange with the ocean it isn't at all unusual for the temp to vary 5-10 F (or even more) over the course of 24 hrs. In fact, the temp. may fluctuate that much in less than an hour as the tide comes back in and cools down an area. On forereefs the temp. tends to be pretty stable over the course of a day since there is constant mixing with the ocean. Seasonally most reefs experience ups and downs in temp. Reefs close to the equator tend to not have as large a seasonal variation as reefs further from the equator, but it's still noticable. For instance, in Belau the winter temp might average 82 F while the summer temp averages 86 F. In the southernmost part of the GBR the temp. might average 74 F for a couple months in winter and 84 F during the summer. Most corals calcify and grow slower in the winter than during the summer, though they do still grow. How much slower depends on how cool it gets during the winter. Corals in Belau probably don't experience a very dramatic difference in growth between seasons while those in the southern GBR definitely do.
I think that's probably fine to have some fluctuation. I've never had problems with daily fluctuations of up to 4-5 F (e.g., min. 79-80, max 84). I would try to keep the daily fluctuation from exceeding this as I don't think allowing the temp. to fluctuate more than this is necessarily a good thing. Is it tolerable? Yes, most definitely. Is it stressful and less than ideal? Probably. Something to keep in mind is that nature does not necessarily offer an ideal environment. There are storms, freshwater deluges, extreme low tides, slack waves from doldrums, etc. which all kill corals. Yes, corals and many animals can tolerate environments like reef flats with lots of daily variability, but they may be growing in spite of these conditions, not because of them. For that reason I wouldn't advise draining a tank twice a month and letting the corals fry under the halides for an hour, even though corals in nature might tolerate similar conditions. Having said all that, a little variation with a range of say 5 F daily is fine IMO. Also, a little fluctuation tends to desensitize corals to bigger swings if something goes wrong (chiller fails, AC breaks, etc.). Sometimes folks try to keep the temp. rock solid (e.g., 78+/-0.1 F) only to find that when the temp. does spike for whatever reason temps. that are normally tolerable (e.g., 84 F) cause severe bleaching. A mean temp. +/- 1-2 F daily is fine IMO and maybe even desirable.
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