10°C (50°F) in freshwater, 100% saturation is at 11.29 mg/L while at 20°C (68°F) at 20 g/L salinity, 100% saturation is at 6.19 mg/L.
Target values: Target DO is typically 6–15 mg/L or >90% saturation (Table A2.2); even if fish can tolerate lower levels, the nitrifying bacteria in biological filters require >80% saturation. Values <2–4 mg/L are likely to cause morbidity and mortality in most species.
Practical considerations:
Low DO is a common cause of morbidity and mortality in fish. It is often due to poor water flow (e.g. because of a pump failure or in a closed transport container), reduced photosynthesis, and high organic loads. It is more problematic in warm water, because of lower solubility, and for pelagic and reef fish, because of higher oxygen demands.
High DO is rarely a problem, although oxygen toxicity and gill damage are theoretical concerns. A safe upper limit may be 120–140%, although much higher levels are commonly used during commercial fish transports.
DO can vary significantly across the day, particularly when vascular plants, algae, or phytoplankton are present, as these produce oxygen during daylight through photosynthesis but consume oxygen overnight. In these systems, DO values are lowest at dawn (Figure A2.2).
Further discussion of low DO is provided in Chapter C1.
Table A2.2 Possible water quality parameters for some common species and groups.
| Units | Koi, goldfish, or similar freshwater species | Tropical marine fish and elasmobranchs | Tropical marine fish and corals | |
|---|---|---|---|---|
| Dissolved oxygen | % | >90 | >95 | >95 |
| Total gas pressure | % | <100–105 | <100–105 | <100–105 |
| Temperature | °C | 15–22 | 22–28 | 22–28 |
| Salinity | g/L | <0.5 | 28–35 | 32–35 |
| Unionized ammonia | mg/L | <0.02 | <0.02 | <0.02 |
| Nitrite‐nitrogen | mg/L | <0.1 | <0.1 | <0.1 |
| Nitrate‐nitrogen | mg/L | <50 | <50 | <15 |
| pH | — | 6.5–7.5 | 8.0–8.5 | 7.5–8.5 |
| Alkalinity | mg/L of CaCO3 | 50–150 | >200 | >200 |
| Hardness | mg/L of CaCO3 | 50–150 | 150–300 | 200–400 |
| Chlorine | mg/L | <0.03 | <0.03 | <0.01 |
| Iodide | mg/L | — | 0.03–0.06 | — |
Total Gas Pressures
Total gas pressures (TGP, or total dissolved gases, TDG) include the partial pressures of oxygen, nitrogen, carbon dioxide, and argon in the water. The solubility of each gas depends on its properties as well as temperature, salinity, and total pressure. High total gas pressures (supersaturation) occur when the total of the partial pressures is greater than the atmospheric pressure. Supersaturation can cause gas emboli and associated pathology in the vasculature, eyes, or subcutaneous spaces in fish (Smiley et al. 2011). While all gases can show increased partial pressures, nitrogen is the most likely to cause gas emboli because its insolubility makes it more likely to come out of solution within the fish.
Frequency of testing: It is useful to obtain baseline gas pressures for every system; this provides a comparison if issues are seen. Gas pressures may be monitored continuously in intensive aquaculture systems.
Sampling: Gas pressures must be measured on site in the system and represent only that moment in time.
Testing: TGP meters, also known as saturometers, are used (e.g. Point Four™ Tracker, Pentair Aquatic Eco‐Systems, Cary, NC). TGP meters must be kept in the water for >30 minutes as it takes time for the gases to diffuse across the membrane and produce an accurate result; the initial results need to be discarded (Figure A2.3). It is most useful to track results over at least a 24‐hour period to look for transient increases.
Units: TGP is provided in milligrams per liter (mg/L). It is simultaneously reported as percentage saturation (%) relative to atmospheric pressure of air at the water surface.
Figure A2.2 Changes in dissolved oxygen (DO), carbon dioxide (CO2), and pH across the day due to respiration and photosynthesis.
Figure A2.3 Total gas pressure meter readings from a fish system showing the initial values that are related to calibration and should be discarded.
Target values: TGP should ideally be 90–100%, but <105% is a common target (Table A2.2). Values >110% are likely to cause pathology.
Practical considerations:
High