I am looking for the article I mentioned earlier but can't find it.
Here is an article I just found that discusses at length average reef temps, and the notion of stability generally.
and here is another one... with charts!!!
Now don't get me wrong, I am not advocating that you allow your tank to have such large fluctuations and this is due to a couple of factors.
The first is that very few of us keep corals from one area of the world, and from one location on the reef. Our corals come from many different areas of the world and from different locations on the reef. Because of that you can't assume that any one particular coral is going to be able to survive a large swing in temp.
The second reason is that our systems are far from pristine. A perfectly healthy and robust coral may be able to survive a six degree swing in temp, but a slightly stressed coral may not. As our corals are kept in a captive environment, and we do not have the same margin of error as their natural environment I believe (based upon a decent amount of experience) that we are better off keeping things as stable as possible. Due to the closed environment, we will inevitably have problems that will stress our corals, some of these problems we may never know about (when is the last time you tested for strontium). While your corals may survive a large temp swing one day, next week may be a different story.
All that being said, if you have problems with temp fluctuations, I believe (and it is nothing more than a belief, albeit based on experience) that you should consider slowly raising your tank temp to avoid a large swing in temp. High temps are not the problem, it is the overall swing in temp that is the killer.
Here is an article I just found that discusses at length average reef temps, and the notion of stability generally.
and here is another one... with charts!!!
Now don't get me wrong, I am not advocating that you allow your tank to have such large fluctuations and this is due to a couple of factors.
The first is that very few of us keep corals from one area of the world, and from one location on the reef. Our corals come from many different areas of the world and from different locations on the reef. Because of that you can't assume that any one particular coral is going to be able to survive a large swing in temp.
The second reason is that our systems are far from pristine. A perfectly healthy and robust coral may be able to survive a six degree swing in temp, but a slightly stressed coral may not. As our corals are kept in a captive environment, and we do not have the same margin of error as their natural environment I believe (based upon a decent amount of experience) that we are better off keeping things as stable as possible. Due to the closed environment, we will inevitably have problems that will stress our corals, some of these problems we may never know about (when is the last time you tested for strontium). While your corals may survive a large temp swing one day, next week may be a different story.
All that being said, if you have problems with temp fluctuations, I believe (and it is nothing more than a belief, albeit based on experience) that you should consider slowly raising your tank temp to avoid a large swing in temp. High temps are not the problem, it is the overall swing in temp that is the killer.
Its also hooked up to a controller 
