If oxygen levels drop low, fish become stressed. Stress can then trigger secondary problems, such as poor growth, poor reproduction and diseases. So it is vitally important to maintain adequate amounts of oxygen in the water. The amount of oxygen needed depends on the kinds of fish in the pond. When oxygen is less than five ppm, fish may show signs of stress.
Fish kills can occur when oxygen is less than three ppm.
Oxygen levels below one ppm are catastrophic. Some fish species survive low oxygen concentrations better than others. Unfortunately, these are usually undesirable species, such as common carp and bullheads. It is a relatively simple test using pre-measured chemical powders. Another chemical feature of your pond is pH or acidity.