The importance of oxygen
Lack of dissolved oxygen in a pond kills more fish than any other reason. It can be detected by fish gulping at the surface as they desperately try to suck in more oxygen. At lower levels fish become lethargic and go off their food. As with all stressful conditions they become far more susceptible to disease and predation.
Simple test conditions are available for most water quality parameters such as ammonia, nitrate and pH. Although there are some available for measuring dissolved oxygen a much more effective method is to use an electronic hand-held meter. These hand-held meters require a bigger initial investment but their running costs are much lower and it is a simple operation to record this vital parameter at several points around a pond system.
Oxygen is also essential for the health of a biological filter and any plants that may be present. It is so important in all aquaponic systems.