Most aquarists with reef tanks understand that maintaining their corals within the proper temperature range is important in order to keep them healthy and growing. If the temperature is too low or too high, the zooxanthellae algae, which most corals require for survival, will die or vacate the polyps. When the algae leave the coral, it exposes the white base calcium carbonate of the coral, it is sometimes called coral bleaching. You may recall the news stories about coral bleaching in some of the world's coral reefs, which reportedly began in the early '90s.
The National Oceanic and Atmospheric Administration states that the optimal range for coral to thrive is between 73 and 84 degrees F. So it is probably best to keep your aquarium well within this range to start.
With such a wide temperature range in which coral can thrive, what is the best temperature for your reef tank? Perhaps the best way to make a decision is to look at what the temperatures were when your corals were in the wild. If you look at Coral Reef Regions in the World, you will see where corals grow in the wild. If you compare this to the indicated temperatures in NOAA's Sea Surface Temperatures, you will see that a vast majority of coral reefs are found where water temperatures are between 80 F and 89 F and actually into the lower 90's in the Red Sea.
The Indo-Pacific, Caribbean and the Red Sea are where most of the corals in the aquarium trade are collected. Perhaps the easiest way to find out is to ask when you purchase your corals where the corals were shipped from.
Since most aquarium corals are collected from the Indo-Pacific and the Caribbean where the water temperatures are routinely between 85 and 89 F, perhaps you may not need to get too concerned as long as your tank temps do not rise above or well below these levels.
There are some potentially serious problems with higher water temperatures in saltwater aquariums, however. The higher water temperatures in saltwater (and freshwater, too), the less dissolved oxygen (DO) it will hold, which is detrimental to every one of the organisms in your tank. In general, dissolved oxygen levels are about 20% less in seawater than in freshwater.
Without getting into all of the scientific calculations, formulas, and data, let's just accept that the saltwater at the equator, where the surface water temperatures are in the mid to upper 80's, holds about two-thirds of the DO as the water at the poles, where the water is really cold.
Different fish species require different levels of DO. As an example, the Clownfish requires a Dissolved Oxygen level of about 7 mg/l while the Marlin requires a DO of about 3 mg/l. Ironically, most of the critters in saltwater aquariums require higher levels of DO than do most of the other fish in the ocean. In the wild, most of the saltwater tropical fish are found on or near the reefs, as opposed to the deeper waters of the oceans, where the DO is higher due to the waves breaking on the reef, aerating the surface water.
Higher water temperatures also increase the rate at which material decomposes in saltwater. Bacteria increases its reproduction rate, which increases the consumption of O2, which lowers the DO level in the water.
Most of the saltwater tropical fish and invertebrates that we have in our marine aquariums originate from waters in the wild where the temperatures average in the low to mid 80's so this would be a good target temperature for our tanks.