I still think that disolved oxygen is a big issue for our (generally overstocked) tanks. As temp increases, so does the metabolism of the oxygen-consuming animals. But the solubility of oxygen in the water goes down. So the tank is getting closer to "the edge". One small glitch (power outage, pump failure, etc) and a higher-temperature tank is more likely result in suffocation of the inhabitants than a similarly-stocked tank at a lower temperature.
Reefs, on the other hand, are lightly-stocked. And have huge influxes of "new" water that is not subject to a pump failure.
I am not saying that a reef tank cannot be perfectly healthy at a temp in the upper 80's. It is just more at risk of a crash.
We do a lot of things to balance the risks in our tanks. For me, shooting for a lower temperature is one of them.
Heres a quote from Greenbean on the oxygen issue from this thread
http://www.reefaquariumforum.com/reefkeeping-myths-t3397.html
The granddaddy of all reefing myths- Temperature!
The ideal temp for reef tanks is about 77-80
Well this certainly isn't a bad range, but it's not necessarily ideal either. The worldwide average for coral reefs is a wintertime low of 77 to a summertime high of 86. The overall yearly average is 82. The average temp in the coral triangle here reef diversity is highest (and the majority of the livestock in the hobby is collected) is around 82-83 depending on the source. The thermal optimum, which is the temperature where a species grows best, has been tested for a handful of corals and for almost all species falls between 82-84.
Cooler temperatures are better because they give you more margin of error in case of an emergency
The threshholds for thermal damage in corals are set by acclimatization. The rule of thumb is 2-4 deg F above the normal seasonal maximum temperature. Whether your tank normally maxes out at 78 or 86, the corals will still only handle prolonged exposure to about 2-4 degrees above that.
Sometimes oxygenation of the water is cited as contributing to this effect too. While this is somewhat true, the effect is very minor. Increasing the temperature from 78 to 86 only reduces the oxygen saturation point by 7%. That still leaves you at about 300% of the safe level of oxygen. The temperature effect on metabolic demand for oxygen does not follow a clean curve in the sense that you can say higher temperatures demand more oxygen as people often insist. Again, this has to do with acclimatization and can get somewhat complicated.
The temperature on reefs is stable
Not by a long shot. A typical reef varies at least 3-8 degrees per day with some varying as much as 15. Because these were only measured over fairly coarse time periods, it's likely that short-period changes that occurred quickly were missed. These are not slow changes occurring as the sun heats the water either. In fact it has been noted that the minute-to-minute variation is frequently as much as half of the yearly variation. The origin of these fluctuations are shifting currents, tides, and internal waves. As a result, variation actually increases with depth, contrary to what most hobbyists might imagine.
Stable temperatures are essential for healthy corals and fish
This one seems to have originated with studies done in temperate freshwater fish that showed increases in disease when they were exposed to rapidly fluctuating temperatures. The same has not been demonstrated for tropical marine fish, much less for corals. Given the unstable nature of wild reefs, you would not expect this to be true for reef organisms. In fact, it has been noted that larger fluctuations help protect corals from temperature stress.
