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Most fish are poikilothermic and have preferred temperature ranges. Water temperature impacts all aspects of metabolism, behavior, appetite, water chemistry, disease dynamics, drug pharmacokinetics, and sensitivity to toxins. Because cooler water is heavier than warmer water, steep temperature gradients (thermoclines) can develop in deep habitats.

Frequency of testing: Temperature should be assayed continuously or at least daily. It should be measured when fish are moved between bodies of water to identify any temperature differentials; these may affect acclimation needs. It should be measured during restraint and ideally during transport.

Sampling: Temperature must be measured on site in the system.

Testing: A variety of thermometers are available, both independent and integrated into heat exchangers. They may be contact or noncontact (e.g. infrared thermometers). The meters should be certified (e.g. by the National Institute of Standards and Technology [NIST] in the United States) with minimum and maximum temperatures also reported. Accuracy and precision should be checked for each thermometer as quality can vary widely.

Units: Temperature is reported in degrees Celsius (°C) or Fahrenheit (°F).

Target values: Species preferred temperature zones vary, but some approximate guidelines are:

 Tropical reef fish: 22–28°C (72–82°F).

 Goldfish (Carassius auratus), koi (Cyprinus carpio koi), similar temperate species: 15–22°C (59–72°F).

 Atlantic salmon (Salmo salar), rainbow trout (Oncorhynchus mykiss), and other similar cold‐water species: 7–18°C (45–64°F).

Practical considerations:

 Water temperature needs to be within the preferred range for all animals in the system. This can be difficult with populations that include species with different preferred ranges, e.g. bonnethead sharks (Sphyrna tiburo) are often housed in large tropical reef systems at 23–24°C (73–75°F); while this suits many tropical teleosts, bonnetheads often do better at slightly higher temperatures.

 Temperature problems are most likely when the water temperature differs significantly from the ambient temperature (e.g. cold‐water aquarium in a warm room). These animals are at high risk if there is a mechanical failure of the life support system and the water temperature becomes closer to ambient temperature.

 Temperature changes within a system are often due to inadequately sized heat exchangers, heat exchanger malfunctions (e.g. loss of power), water additions (e.g. rainfall, cold tap water), or changes in ambient temperature (e.g. during shipping or severe weather).

 Rapid changes are more likely to negatively impact fish health than slow changes. In general, water temperature should not change by >2°C/h or 5°F/h, although sensitivity varies. The most common event that might expose fish to rapid water temperature changes is when they are moved between water systems without an acclimation period.

 Handling fish under chemical or manual restraint when ambient temperature differs significantly from water temperature is another consideration. The temperature of the water for ventilating and moistening fish can easily change through the procedure and this should be monitored and managed.

 If water temperature increases in a system with high ammonia or heavy metals, their chemical equilibrium shifts to the more toxic forms and morbidity and mortality become more likely.

Further discussion of temperature stress is available in Chapter C1.