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Dissolved oxygen (DO) is critical to support animal and microbial respiration. Low DO is a common cause of acute morbidity and mortality in fish, although species sensitivity varies (see Chapter C1). Adequate microbial respiration is essential for the biological filtration within a life support system. Since low DO can damage the microbial population supporting biological filtration, it can lead to increases in toxic nitrogenous wastes.

Frequency of testing: DO should be assayed routinely where possible (e.g. daily). DO may be monitored continuously in intensive aquaculture systems or with sensitive species. At a minimum, baseline values should be obtained for each system. DO can vary across the year, particularly for surface water, so baselines should be established across all seasons. DO should be checked prior to and during any immersion treatment, restraint, or transport.

Sampling: DO must be measured on site in the system. The result represents only that moment in time and values can change rapidly. DO should be measured in multiple locations, as it will vary with depth, water flow, and surface water agitation.

Testing: DO is measured using DO meters (Figure A2.1). Most consist of an oxygen‐permeable membrane and use an oxygen‐dependent chemical reaction; polarographic or galvanic electrodes measure the electric current produced. Polarographic models require the probe to be moved through the water. Fiber‐optic units measure the effect of oxygen on fluorescence. Calibration is required each time a meter is turned on. Most units are calibrated against moist room air, but the instructions in the user manual should be followed. The membrane should be replaced periodically and must be changed if there are tears or bubbles.

Table A2.1 Common advantages and disadvantages of different water sources.

	Advantages	Disadvantages
Municipal water	Availability Consistency Low risk of pathogens Usually high DO Usually low H2S and CO2	High chlorines and chloramines May have high nitrates, phosphates, or heavy metals Cost
Surface water	Good conditions for endemic species Usually low H2S and CO2	Need to be physically close to the source Variable quality; changes may be seasonal High risk of pathogens High risk of toxins (e.g. harmful algal blooms, pesticides) High risk of turbidity
Ground water	Consistency Low risk of pathogens Low risk of environmental pollutants Low risk of organic matter	Variable availability Often low DO and supersaturated with N2 Often low pH and high CO2 Often low temperature Potentially high H2S and Fe

CO₂: carbon dioxide; DO: dissolved oxygen; Fe: iron; H₂S: hydrogen sulfide; N₂: nitrogen.

Figure A2.1 Dissolved oxygen meter.

Units: Oxygen is measured in milligrams per liter (mg/L), equivalent to parts per million (ppm). It is simultaneously reported as percentage saturation (%). This is because the amount required to saturate water varies with temperature and salinity. For example, at 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	—