The First Limiting Factor of Water Quality
Air breathing animals are used to air containing about 21 percent oxygen, whereas aquaculture is conducted in water containing less than .0001 percent oxygen (10 mg/L). With so little oxygen available in the best of conditions, it is apparent that knowledge of dissolved oxygen (D.O.), oxygen measuring and aeration equipment is very important to the aquaculturist.
If the oxygen level is too high, oxygen supersaturation can cause gas embolism, depress metabolism and inhibit respiratory enzymes, all of which can kill fish. If it is too low, the fish may not eat and may expend additional energy seeking oxygen. If lower yet, they can experience severe stress and, of course, death.
The diurnal oxygen cycle in outdoor ponds is show below. The delicate balance of dissolved oxygen vs the rate of oxygen consumption can shift rapidly with changing algal, temperature and wind conditions. In outdoor ponds, lower D.O. levels will occur in the summer because the rate of oxygen consumption increases as the temperature increases. Oxygen is THE MOST IMPORTANT thing to monitor. Monitoring and record-keeping will provide predictive knowledge.
Measure oxygen after altering water flowrates, feed rates, etc., and when developing new growing systems. No one can just look at the water and know the oxygen level. Oxygen can only be measured with a test kit or, more conveniently, with an oxygen meter.