What is the ideal water temperature for a reef tank?
Well, the truth is there is no one standard answer. For the most part, I agree with the Hauters that it depends on the origin of the creatures you have obtained in your tank.
Some of the critters you have obtained originate from the shallows of the Indo Pacific Ocean and are used to temperatures in the upwards of the mid 80F, over 30C. While other critters comes from slightly deeper areas, or borderline sub tropical region that they really do best at slightly chiller water.
An example is the popular Catalina goby fish. They are from the Eastern Pacific coast of the Catalina Islands and are used to water temperature no warmer than 72F. Now imagine mixing this fish with a Hawaiian yellow tang where water temperature is at constant 78-82F. At least one of the two fishes will find stress depends if you go the warm route or the cool route.
I have previously ran my tanks at a standard of 25C. After reading Shimek’s paper, along with my personal snorkeling experiences in the waters of Hawaii, Belize, and Taiwan reef, I decided to raise the temperature up to at least 27C to encourage better metabolism among the corals.
Above is a thermostat control with built in heater I purchased in Taiwan. This heater used advanced sensor to remotely control the heater and maintain the set temp. Currently it is set at 26C. I plan on maintaining this temperature for a week before moving it up one more degree.
How do I know what water temperature to keep? Well, one easy way is to learn the origins of your creature. If they are labeled Australia or Indonesia, chances are they are used to warmer water (but you obviously don’t want to keep them in bleaching induction condition). So I would suggest you keep the temperature at or around 80F (27C). If your critters are from the Gulf of Mexico, Eastern Pacific, Hawaii, or Japan, you may want to research the specific region’s annual mean temperature.
One example is the Swallowtail angelfish genicanthus watanabei. They are from the deep waters of Japanese and Western Pacific so they can endure temps in the low to mid 70s, but are probably not as happy in the 80s.
Some signs of your corals disliking the water temperature (assuming all water parameters are correct): lack of polyp extension, color fades or bleach, tissue degeneration. Temperatures higher than tolerable range will cause fish to breath excessively. You will observe them try to breath from the water surface as higher temperature can lead to lower dissolved oxygen.
Temperatures too low will cause the fish to appear lethargic, go into hiding, lost of appetite. And with corals you may see it slowly wither away as the animal isn’t able to fully metabolize.
Personal opinion, I think the temperature posted by Liveaquaria is a “quarantine” and “acclimation” protocol. You may use the 72-78 temperature range to acclimate critters to your tank. But having swam in most of the world’s natural, healthy reef system, I know as a fact that the water temperature is not consistent. You’s soak in a mean temperature of 78F but with pockets of really warm water in the mid or upper 80s brushing by from time to time. So I think it is completely OK to set reef tank temperature between 78-82, but I will take caution for anything warmer. After all, they are not discus.