The temperature issue is one of great disagreement among many reefers, and there is no simple answer to your question because it is sure to spark a debate. My opinion is that the best way to maintain animals in a reef tank is to try our best to duplicate the habitat from which they were collected, and for the vast majority of coral reef species that means something on the order of 35-36ppt (roughly 1.026-1.027) salinity and 82-84F. You'll never see that recommendation in any book or on the back of a bag of artificial salt, and the reasons people have for maintaining unnaturally low temperatures and/or salinities are varied. In your case, you ask how to get you tank back to "normal" but 83-84F is an average temperature for most natural coral reefs, and daily fluctuations in temperature (often on the order of 5F, see for example the temperature data compiled by DBW from the GBR) are normal and harmless. Ron Shimek has a detailed article explaining this in the Nov'97 Aquarium Frontiers. Ron explains that the depth argument is not a good one because surface seawater temperatures typically extend to roughly 100ft in tropical areas, and we rarely see animals in our tanks from deeper areas than that. I recently explained why the oxygen concentration argument is invalid in my TFH column, and I'll reprint the appropriate section below:

The amount of oxygen which can dissolve in salt water depends on the exact salinity and temperature of your aquarium -- the table below gives you the values of oxygen saturation (mg/l) across a wide range of temperatures and salinities. The basic answer is that saturation for an average marine aquarium is roughly 6.5 mg/l, but that this value means relatively little to the overall health of your animals (which I will explain in detail below) -- (I can't figure out how to get this table to work properly, sorry...)
Salinity ( 0/00 – parts per thousand)
Temperature (oC) 0 10 20 30 40
10 (~50 F) 13.0 12.2 11.4 10.6 9.8
15 (~59 F) 10.3 9.7 9.2 8.6 8.1
20 (~68 F) 9.4 8.8 8.4 7.9 7.4
25 (~77 F) 8.5 8.0 7.6 7.2 6.7
30 (~86 F) 7.8 7.4 7.0 6.6 6.2
At this point it makes sense to ask whether out tanks need to be at saturation, and what normal levels of oxygen are in the ocean in areas around coral reefs. It makes sense that surf zones are probably close to saturated with oxygen, but what about the areas around coral reefs – how much oxygen is in the water there? Well, natural seawater in areas around reefs are generally between 95-110% saturated with oxygen, and the majority of areas measured are at 100% saturation, so it seems reasonable that level of oxygen which can dissolve in seawater may be important to an aquarium. Obviously oxygen is important to our animals, and if water around natural reefs are usually saturated with oxygen, then perhaps that argument has some merit?

I don’t think so.

Measurements around one Caribbean island showed that night time oxygen levels range from about 5.0-6.5 mg oxygen/l, and daytime levels rise to around 7.5-9.0 mg oxygen/l (saturation was roughly 6.25mg/l) depending on the reef measured. Daytime oxygen concentration rise to above saturation because photosynthesis produces oxygen at a higher rate than it can leave the water by diffusion, and the areas of lower oxygen concentration are actually those with the most gas exchange, because oxygen diffuses out of the water more rapidly and is closer to the saturation level of about 6.25mg/l than areas with limited gas exchange. The opposite is true at night, when respiration uses up available oxygen and the areas with the lowest gas exchange depress the oxygen concentration farthest below saturation. Walter Adey reports similar levels of oxygen in his ATS-based tanks at the Smithsonian, but is also quick to point out that it is not the oxygen concentration itself that is important but rather the exchange rate of oxygen that is important to the health of the animals.

Let's say you start with an average well-stocked 50G reef aquarium. The respiration of the animals in the aquarium would likely be on the order of 3g of oxygen per hour. Even if you could supersaturate the seawater (lets say it's at 9mg oxygen/l -- the highest recorded on the natural reefs I mentioned), that still only gives you about an hour before the animals suffocate if gas exchange is limited. Of course, protein skimming, turbulent water flow (e.g., "dueling" powerheads, and especially surge devices), and photosynthesis will alter that rate of exchange, and with the normal exchange rates of roughly 4-6g of oxygen per square meter of surface area per hour, the respiratory needs of your animals should be easily met. While it is true that both salinity and temperature will affect the particular value of the oxygen saturation coefficient in seawater – e.g., if you're being good and maintaining your tank at a natural salinity of ~35-36‰, then the oxygen saturation at 75F (~24C) is roughly 6.8mg/l, while at 82F (~28C) it drops to only 6.6mg/l) – this is pretty much a non-factor. Given that natural reefs can fluctuate in oxygen concentration from around five to around nine on a daily basis, it seems a little silly to me that anyone should worry about a difference of 0.2 mg/l in the middle of that range. If this small difference in oxygen saturation point makes any difference to the inhabitants of your tank, you have other problems, and must be seriously under-circulating or overloading your tank! As long as your dissolved oxygen level remains above about 5 mg/l then there is no reason for concern with the oxygen level of your tank, and even if there were, it is the rate of gas exchange in the aquarium rather than the total amount of oxygen that is able to dissolve in seawater at saturation that is of concern to the health of our animals...

Rob