An estimated 20,000 species of fish live in the sea, from tropical reefs topping 80 degrees to depths that are always near freezing. For coldblooded fish confined to saltwater aquariums, temperature control is critical.
Warm-water species thrive at temperatures ranging from 75 to 82 degrees Fahrenheit. Many experience stress and might die if the temperature goes below 70 or above 90 degrees.
Fish that inhabit temperate waters do best at temperatures from 50 to 70 degrees. Tide-pool species, which are used to slightly higher temperatures, might adapt to warmer water.
Maintaining a temperature between 78 and 82 degrees is ideal for most tropical species. Two or three small heaters might deal more reliably with occasional operational glitches than a single large one can.
Aquarium chillers are necessary for cool-water species, especially in summer, and to temper heat generated by pumps and lights. In an emergency or in cool weather, a small fan directed over the surface of the water can be sufficient. Extreme situations can be helped by a immersing a plastic bag or container of ice.
More important than actual temperature, rapid changes in water temperature can be fatal. If a heater or chiller fails, correcting the temperature slowly—about five degrees per hour—allows the fish to adapt.