Hypothermia (the condition of being too cold) is usually the result of an aquarium or pond that is inadequately heated. This serious condition is most common in colder climates or during the winter months. In most cases, a heating unit is either lacking, improperly functioning, or of insufficient strength to maintain water temperatures at optimal levels.
Optimal aquarium or pond temperature is difficult to define because of the wide species variability that exists in the pet fish hobby. As a general rule, most tropical freshwater and marine fishes require temperatures between 75 to 85 degrees Fahrenheit (24 to 29 degrees Centigrade). Temperate and cold water fishes have lower temperature requirements; goldfish and Koi thrive at water temperatures between 65 and 75 degrees Fahrenheit.
The most common cause of hypothermia is the absence of a submersible aquarium heater. If a heater is present, its wattage or power must be evaluated, and it should be checked to make sure it is working properly.
Aquaria placed close to drafty areas, even when properly heated, may still experience hypothermia. An accurate thermometer can be used to monitor water temperature and diagnose this problem.
What to Watch For
Fish suffering from hypothermia appear sluggish, depressed, and may be anorectic (not eating). Hypothermia can lead to an abnormal metabolism and dysfunctional immune system. Mortality (death) varies and depends on species, temperature of the water, and duration of exposure.
A quick diagnosis and correction of this problem yields a favorable prognosis in most cases. Your veterinarian can take a history and perform water tests, which can help rule out other problems that may mimic hypothermia.
In most cases you should be able to diagnose hypothermia with the aid of an accurate thermometer and, perhaps, the assistance of your pet store clerk. If your fish are suffering from hypothermia, the temperature of their water should be gradually increased slowly, at a rate of no more than 5 degrees Fahrenheit per hour.
Most closed, recirculating aquariums can be thermally maintained with one watt of heating power per liter of water (4 watts/gallon). A thermometer should always be present in the aquarium in order to maintain optimal temperatures.
Most heaters are equipped with a thermostat in order to control and maintain environmental temperature tightly.