Fish and invertebrates are cold-blooded creatures and can’t just lower or raise their body temperatures to accommodate to the surrounding temperature. This is why an aquarist has to maintain an optimal tank temperature. The answer to the question of what temperature is optimal is not a simple one. Knowing what kind of organisms are in an aquarium and where they came from is important when deciding on water temperature.
Aquarists have different views on the issue of optimal temperature. Some believe an optimal temperature is between 75 and 77 degrees whereas others believe in going for temperatures as high as 80 to 85 degrees. Some scientists believe that aquarists should rather keep separate systems for organisms from different geographical areas.
How high is too high?
At too high a temperature, inhabitants coming from colder water temperatures are stressed. Some aquarists believe it is safe to increase water temperature under certain conditions – no sign of algae, an efficient way to remove increased waste and healthy corals in a stable tank.
However, algae can bloom uncontrollably with higher temperatures, and there are coral and fish diseases that thrive with higher temperatures.
As the temperature increases, the level of dissolved oxygen in the water decreases. Ironically, many organisms in saltwater aquariums need higher levels of dissolved oxygen because they are found on or near reefs. The dissolved oxygen content of the water there is higher as waves break on the reefs and aerate the water. For example, a clownfish requires much more dissolved oxygen than a marlin.
When fish start dying from lack of dissolved oxygen, they release ammonia which is damaging to the corals. The problem of lack of dissolved oxygen can be addressed by good water movement, enough surface area, and good tank aeration.
Drastic changes in temperature
Any drastic temperature changes are harmful. Coral need zooxanthellae algae for nutrition. Sudden changes in temperature force zooxanthellae algae to abandon the coral and it bleaches and eventually dies.
Any quick changes in temperature are harmful to most creatures causing stress and eventual death. However, gradual acclimation usually allows them to adjust. Constantly changing temperatures up and down seems to be even worse than a high temperature.
If you have problems with temperatures that fluctuate more than four degrees up or down, one solution is to install a chiller. A chiller basically cools your aquarium water when the house is too warm. This allows you to maintain stable temperature settings.
A good target temperature
Maintaining an optimal and steady temperature is essential to tank inhabitants and presents a challenge for many aquarists. Most of the invertebrates and tropical fish in our saltwater aquariums come from waters where temperatures average in the low to mid-80's. It makes sense that a temperature in this range would provide the largest safety margin. Try to find the sweet spot – not too high or too low – and set up heating or cooling accordingly. A temperature of 78 to 80 degrees appears to be favored by many aquarists.