Brown Jelly disease is common with temperature over 80F , Corals do not expand as well with temperature over 80F.
These are both anecdotal. There has been some association shown with some diseases and temperature, but these are elevated temperatures meaning unusually high. 80 isn't even close to fitting that bill. There's no demonstrated correlation between temperature and polyp extension or between polyp extension and health.
Wide fluctuations in temperature are commonly associated with Cryptocaryon irritans infections in fish.
Again, this is commonly claimed, but has never been shown. The only association demonstrated between temperature fluctuations and disease in fish has been with FW or temperate fish. Again these are associated with fluctuations the animals wouldn't experience in the wild.
Higher temperature causes lower oxygen levels while also raising fish metabolism (kind of a double whammy).
Yes and no. This is a whole lot more complicated. Higher temperatures do result in lower oxygen at saturation. However, over the range of reef temps, the difference is nearly negligible. Even at 90 deg F, the saturation point is still more than 200% of the safe limit. Reef animals are also highly tolerant of hypoxia as it is a regular occurrence on the reef. Reef fish can get down to 12-20% of saturation with no issues. Below this they actually lower their respiration rate and conserve oxygen. Eric Borneman has done tests on several different setups and found that none of them even approached these low saturation values.
Also, how temperature affects respiration isn't clear. It's not as simple as warmer temperatures= more metabolism. This ignores acclimatization, which we know occurs. This means that if you have an animal that's acclimatized to 77 degrees and one that's acclimatized to 83, they could actually have the same resting respiration rate. We also know that the stress threshold is a product of acclimatization too, not genetics. That means that by lowering the temperature, you are not increasing your margin of error. The safe upper temperature is lowered as well. The rule of thumb is that stress occurs at 2-4 deg F above the normal maximum temp, whether that temp is 78 or 86.
Basically what this means is that yes, there is slightly less oxygen, but the difference due to saturation and changes in metabolism is very small. If temperature ever solves or creates an oxygenation issue, that's a sign of a majorly overstocked tank. Even in emergencies where there is no oxygenation, the difference you can expect to see dependent on temperature is seconds to minutes, not hours.
As for what kind of temperature is best for these animals, it's close to what they're found at in nature. The average yearly range of temps where reefs occur is 77-86 with an overall average of 82. The center of coral diversity has an average temp of around 83. For the corals whose thermal optima (the temp they do best at) has been determined, most fall between 82-84.
Fluctuations are a regular and important part of the reef environment. Typically they're in the range of 3-5 degrees per day, but in or near shallow, enclosed areas like lagoons, or on high latitude reefs they can be around 15 degrees per day. They occur quickly. It's documented that minute-to-minute variations are often as much as half of the yearly variation. There is no indication that they're a source of stress, nor would you expect them to be. In fact they have been shown to be important in moderating the stress response in corals. In other words, fluctuating temperatures make the animals
less prone to stress. Fluctuations are also important in regulating the reproductive cycles of many of our animals.
There is no good reason to believe that keeping a tank either cooler than natural or more stable than natural offers any real benefit.
