High TGPs are often due to entrainment of air into a pump or valve due to a crack or leak upstream, turbulent water flow (particularly when at depth), or poorly designed gas‐exchange towers (e.g. poor water distribution, poor selection of gas‐exchange media, flooded gas‐exchange media, or air entrainment leaving the tower).
Single TGP values are less useful than serial monitoring over time (e.g. over 24 hours); supersaturation events can be transient and may be missed with a sporadic sampling schedule. The impact of corrective action is also assessed using serial monitoring.
Low TGPs are unusual. They may be associated with low dissolved oxygen or low water flow and warrant investigation.
Further discussion of gas supersaturation is available in Chapter C1.
Temperature
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.
Salinity and Salt Composition
Salinity is the concentration of dissolved ions in water. The main ions in seawater are sodium and chloride, with calcium, magnesium, sulfur, potassium, and many other inorganic elements. The total salinity and the concentration of each ion affect behavior, physiology, disease dynamics, drug pharmacokinetics, and sensitivity to toxins. Since ions increase the density of water and make it easier to conduct an electric current, specific gravity and electrical conductivity can be used to indirectly measure salinity.
Marine and brackish water aquariums make use of natural or artificial seawater. Artificial seawater is typically made from dechlorinated municipal freshwater with salt mixtures added. These mixtures may be commercial (e.g. Instant Ocean®, Blacksburg, VA) or customized (Bidwell and Spotte 1985). For commercial mixes, sodium chloride alone and products with anticaking agents should be avoided. For proprietary mixes, the salts should be food‐grade or reagent‐ and analytical‐grade to reduce the risk of contaminants. Phosphates are rarely required in salt mixes for established aquariums as levels accumulate from the food fed. Bromides are less common in salt mixes now, as they do not appear to be required at the levels seen in natural seawater and can form harmful residual oxidants with ozone disinfection. Salts should be mixed in freshwater or brine in a dedicated, off‐exhibit tank and then tested, at least for salinity; salts should not be added directly to an aquarium system. Salt mixtures are often a significant part of the cost of marine aquariums.
Freshwater aquariums may use municipal, surface, or ground water. Some freshwater fish do best in low‐ion water; in these cases, ions may be removed from freshwater by deionization, reverse osmosis, or distilling.
Frequency of testing: Salinity should be assayed regularly; this often includes testing incoming water routinely or with each production batch, and testing each system every two to four weeks if stable. A full cation analysis (e.g. concentrations of Na+, Ca2+, Mg2+, and other ions) should be considered periodically, both as quality control for artificial salt water and in long‐established closed systems to check the ion balance and allow correction if needed. Salinity of transport water should be measured prior to acclimation to identify differences from the destination water; this may affect acclimation needs. For high‐risk fish transports, a full cation comparison is recommended prior to transport.
Sampling: Standard sampling is described in Box A2.1. Samples can be stored for a few days at room temperature or weeks when refrigerated or frozen.
Testing: The simplest test uses a refractometer or hydrometer to measure specific gravity; this measurement of density reflects total dissolved solids rather than true salinity, but gives a quick approximation of salinity. Refractometers need to be calibrated prior to each use and some require temperature correction. Handheld conductivity meters provide more accurate indirect measurements of salinity in salt water and total dissolved solids in freshwater. Ion‐exchange chromatography is used for cation analysis.
Units: Salinity in aquariums is usually measured in grams per liter (g/L), equivalent to parts per thousand (ppt). Other units include psu (practical salinity units), which are roughly equivalent to g/L values. If specific gravity is used, the results are between 1.005 and 1.030; conversion tables