The key to temp is stability.
No. There is no evidence suggesting that fluctuations cause stress or are otherwise harmful to reef animals, nor would you expect them to be. Large, fast fluctuations are regular events on reefs at all depths where ornamentals are collected and these are the conditions our animals evolved in. Stability provides almost no benefit and in fact, may reduce their fitness.
The higher temps are OK too, but for only a few months....like they'd see in the ocean. A sustained, maintained higher temp will usually shorten the lifespan because it increases the metabolism of all involved.
Maintained high temperatures will shorten lifespans for animals with senescence that don't modulate their metabolism (which excludes a lot of the animals we keep). We're talking about normal temperatures though, which range into the mid 80s. There will be no appreciable difference in the lifespan of any of our animal over the range of average reef temps (77-86). It's also not necessarily true that higher temperatures increase metabolism in poikilotherms. It's a lot more complicated than that because they do modulate their metabolisms.
Keep in mind that some fish and corals are used to the lower temp range and may suffer. The reason the 75-78F is touted, is that is is usually safe for most animals sold in the trade.
There's no reason to believe that this is true either. We know the average range of reef temps and the overall yearly average (77-86 and 82 respectively). We also know the optima of many hard corals, which is 82-84 for most species. We also know the average temperature from the center of reef biodiversity (which is also where the heaviest ornamental collection is), which is 83-84. These temps are the ones that are safest for most animals in the trade. Few animals are coming from areas that are in the 70s year-round. There's nothing safer about exceeding the lower thermal limit of these animals than exceeding the upper limit. Both are sources of severe stress and whole reefs have been wiped out in extremely cold periods just as they are during extremely hot periods.
This also assumes that these animals have a set upper thermal limit, which isn't true. Their limits are set by acclimatization so there is no reason an animal coming from a 78 degree reef is any less suited to an 82 degree tank.
My "wisdom" on the issue is that people in the hobby have accepted that 78 is the ideal temperature, but very few stop to ask why that value was picked. For the most part it's based on faulty reasoning that came from a lack of data on the subject of metabolism and thermal stress in reef animals. Almost all of the data is from the last 5 years or so- much more recent than the suggestion of 78. That "ideal" temperature that was produced out of thin air has been a source of a lot of stress and wasted money for many hobbyists. People need to realize that there's nothing harmful or unnatural about temps in the low to mid 80s. If your temp naturally stays in the upper 70s, that's fine. Obviously reefs do quite well at those temperatures. However, if your temp naturally goes to the mid 80s that's fine too. There's no need to spend $300 on a chiller to lower it and you aren't doing your animals any favors by doing so, especially if the chiller fails or the power goes out.
