81 deg F is fine. Even 82 is fine. I personally wouldn't want it getting much above that, but some folks do.
This has more:
Reef Aquarium Water Parameters
http://www.reefkeeping.com/issues/2004-05/rhf/index.htm
from it:
Temperature
Temperature impacts reef aquarium inhabitants in a variety of ways. First and foremost, the animals' metabolic rates rise as temperature rises. They may consequently use more oxygen, carbon dioxide, nutrients, calcium and alkalinity at higher temperatures. This higher metabolic rate can also increase both their growth rate and waste production at higher temperatures.
Another important impact of temperature is on the chemical aspects of the aquarium. The solubility of dissolved gases such as oxygen and carbon dioxide, for example, changes with temperature. Oxygen, in particular, can be a concern because it is less soluble at higher temperature.
So what does this imply for aquarists?
In most instances, trying to match the natural environment in a reef aquarium is a worthy goal. Temperature may, however, be a parameter that requires accounting for the practical considerations of a small closed system. Looking to the ocean as a guide for setting temperatures in reef aquaria may present complications, because corals grow in such a wide range of temperatures. Nevertheless, Ron Shimek has shown in a previous article that the greatest variety of corals are found in water whose average temperature is about 83-86° F.
Reef aquaria do, however, have limitations that may make their optimal temperature somewhat lower. During normal functioning of a reef aquarium, the oxygen level and the metabolic rate of the aquarium inhabitants are not often important issues. During a crisis such as a power failure, however, the dissolved oxygen can be rapidly used up. Lower temperatures not only allow a higher oxygen level before an emergency, but will also slow the consumption of that oxygen by slowing the metabolism of the aquarium's inhabitants. The production of ammonia as organisms begin to die may also be slower at lower temperatures. For reasons such as this, one may choose to strike a practical balance between temperatures that are too high (even if corals normally thrive in the ocean at those temperatures), and those that are too low. Although average reef temperatures in maximal diversity areas (i.e. coral triangle centered Indonesia,) these areas are also often subject to significant mixing. In fact, the cooler reefs, ( i..e. open Pacific reefs) are often more stable at lower temperatures due to oceanic exchange but are less tolerant to bleaching and other temperature related perturbations.
All things considered, those natural guidelines leave a fairly wide range of acceptable temperatures. I keep my aquarium at about 80-81° F year-round. I am actually more inclined to keep the aquarium cooler in the summer, when a power failure would most likely warm the aquarium, and higher in winter, when a power failure would most likely cool it.
All things considered, I recommend temperatures in the range of 76-83° F unless there is a very clear reason to keep it outside that range.
