You asked, "is it ok for my tank temps to vary 82-86 degrees over the day?" Sure.  My tank varies from about 80 to about 86. "what is the max variation and daily high?" This varies with the reef and is primarily due to latitude.  Go to the BTA spawning thread, and see my post where I described finding sea surface temperatures from a given reef.  Pick a reef and model yours after it.The max variation is around 18 F per day, from about 75 to about 93F; recorded at, I believe, Johnston Atoll about 50 years ago.  "i thought bleaching events occurred with sea temps of 82 or a little more." 82F is just about the average temperature for all the reefs in the world.  Some reefs have bleached at that temperature but these have been reefs that were colder reefs, where the normal temperatures peaked at about 80F.  World wide, about 40% or thereabouts of reefs never get as cold as 80F.  Raise your system's temperature slowly, and there will be no problem.  Once it is warmer, remember that for each 2 deg F rise in temperature, the animals' metabolic rates go up by about 10%.  You WILL have to feed more! "if i didn't have to worry about my tank getting to 86 i could probably run it without a chiller saving a bunch of money both short and long term." Definitely.

Cheers, Ron

"The difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind. The love for all living creatures is the most noble attribute of man." Charles Darwin (1809-1882)

You said, "just because these temps are normal today in the wild might it not be better to have a lower temp in our tank? geologically speaking we are in a very warm period. what were reef temps like 10,000 years ago during the last ice age? "

Geologically speaking we are in frigid period, at least the coldest time in the last 250 million years; one has to go back to the Carboniferous periods for any average earth temperatures as cold as ours today... 

During the height of the last ice age, reefs were reduced in latitudinal extent, but the waters of the tropics were not any cooler.  Reefs reached their maximum extent about 40,000 years ago and have been declining ever since, largely due to human impacts. 

"what have sea temps averaged over the last million years?"

This is a meaningless number.  Reefs don't grow at the average sea temperatures today (which is about 40F).  Most of the ocean is cold and dark and well outside of the capability for reef organisms to exist in it.  Read this article and then follow up on the references.  Reef temperatures haven't appreciably changed in the last half a million years or so.  However, even if they had, I would expect that reef animals would adjust their optima through natural selection. 

it seems that if corals start to bleach with only a 2 degree rise in sea temps that they are at or near their physiological limits.

Yes, indeed they are. But, the lower limit is also quite high, so a drop much below their upper physiological limit drops them below their lower one.  A recent article indicates that corals lose about 9% of their calcification capability per degree F as one either rises or drops beyond their physiological optimum. 

Most corals have a growth rate optimum between 82 and 84 F.  Those animals do not begin to show bleaching effects until the temperature rises to well over 88 F, and then only if is prolonged; they will bleach rapidly with a spike of temperature to about 92 or so.  The animals that do bleach at lower temperatures typically bleach at temperatures that are 2 or so degrees F, over the maximum temperature that they normally encounter. 

The bleaching phemonenon is not a new one; however the oceanic temperature rise is.  The temperatures cited as the average in Kleypas et al, were determined by examining measurements taken prior to the oceanic temperature rise.  They determined the average reef temperatue (including a lot of cold water reefs without much coral diversity) to be 81.7 F.  That is probably the LOWER limit for good coral growth and health.

Cheers, Ron

"The difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind. The love for all living creatures is the most noble attribute of man." Charles Darwin (1809-1882)

"wow, that is great news for my tank. those temps should be fairly easy to maintain. most hobbiests i know do not keep their tanks at 82-84 degrees."

Yes that is one of the main reasons most hobbyists don't do well with most animals. 

"the waikiki aquarium which has an amazing tank is only kept at 77 degrees with the aid of a 10 hp chiller and has had amazing growth. they also have a large skylight over the tank, i wonder if local heating effects is raising the temps of the corals directly while the water stays cooler. that could also be a factor in my tank. i guess the tropical sun would do the same thing though."

The Hawai'i reefs are the northenmost tropical reefs in the northern Pacific.  Conventional wisdom states that they are AT or near the lower temperature limit for the animals involved and one of the reasons that the reefs in that area do not contain many types of reef animals comparted to the richer Indo-Pacific reefs is that the water is too cold for them to survive.  There may be some other factors at work, however.  Recent research has shown the waters in that area are also relatively low in carbonate ion, making the formation of coral skeletons difficult, so not only is it cold, some of the necessary substances for survival are in short supply. 

In any case, many animals that do better at more normal reef temperatures will survive at these lower temperatures, but in the long run their metabolic rates are so reduced that they can't respond as well to stresses such as the need to repair injury or to fight off diseases.  They also can't produce enough gametes to overcome the long odds of reproduction.  The dominant corals on many of the cooler water reefs is Pocillopora damicornis, and it turns out to have a real advantage.  Unlike most corals, it has a secondary growth optimum at about 77F even though the main growth optimum is at about 83F.  In other words it grows best at 83 and almost as well at 77, but significantly less well at temperatures in between and outside of those points.

As regards the chillers and the need to keep the animals at a relatively constant temperature.  The "need" for constancy was sort of "grandfathered" into aquarium care.  In the early days of coral reef research, it was assumed that the coral reef environment was a "stable" one having relatively little in the way of variation.  By about 1990 that was shown to be utterly bogus.  If anything, coral reefs vary more in temperature than just about any other shallow water marine environment (far more than the variation seen in the waters off shore of L. A., for example).  Coral reef researchers, such as Prof. Barbara Brown, the editor of Coral Reefs, the journal of the International Society for Reef Studies, put it this way:"...Indeed the constant disturbance and heterogeneity of the coral reef habitat appear to be necessary for the functioning of a healthy coral reef." Brown," B. E. 1997. Adaptations of reef corals to physical environmental stress. Advances in Marine Biology. 31:221-299;

and Rachel Wood in her book (Wood, R. 1999. Reef Evolution. Oxford University Press. Oxford. xi+414pp) states : "Shallow water tropical coral reefs occupy changing and often extreme environments." 

The bottom line is that corals are adapted for and likely require radically changing environmental conditions to meet all of their needs.  The hobbyist fetish for maintianing the animals at "constant" temperatures is not only foolish, it impacts the survival of the animals.  One of the easiet ways to see this is in the lack of reproduction of these animals in hobbyist systems.  Another way is in the abnormal growth forms seen by many scleractinians in reef aquaria.  And finally, although hobbyists tend to think that animals in their systems are doing well - consider that in the real world these are some of the hardiest marine animals known.  Yet, once they enter the reef aquarium environment they become "delicate." 

Well, if you starve an animal and keep it under conditions where its metabolic rate is a bare minimum for survival, and yeah... it becomes delicate.  Of course, the fact that it survives at all is tanamount to a miracle.

Cheers, Ron

"The difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind. The love for all living creatures is the most noble attribute of man." Charles Darwin (1809-1882)