Oxygen is obviously essential to fish health, but how much or how ittle? Two test kits are available, I would recommend owning one of them. There is another test kit at the pet store or here that is in a little vial you break in the water to be tested and collect a blue sample and compare it to a chart.
Minimum levels would be 5ppm. This level will permit fish to live a few days. Levels as low as 3ppm at least partially explain why fish are dying like flies. Levels as low as 7 could, and should be improved with an air stone o r Venturi pump. Levels over 8 ppm are desirable, but 11 or better (up to 14 ppm) are glorious. Low Dissolved Oxygen is an underestimated cause of fish losses, particularly because of its synergistic effects with other toxins like Ammonia.
Hypoxia is 'oxygen starvation'. Most koi ponds are usually well served with venturi retums or waterfalls, but poor pond maintenance, high stocking levels and unusual climatic conditions can lead to low dissolved oxygen (DO) levels. Low DO is likely to occur in summer. As water becomes warmer it can progressively hold less oxygen: and the fish become more active, leading to a greater demand for oxygen; and the bacteria in the pond and filters need more to, as do submerged green plants including algae.
The role of submerged plants and algae should perhaps be clarified. During photosynthesis, submerged plants release oxygen into the water, which is why they are often called oxygenating plants. However, they also respire at the same time, extracting oxygen from the water and excreting carbon dioxide. During daytime they produce more oxygen than they consume, but at night, when photosynthesis ceases, respiration continues and they become net oxygen consumers.
Clearly, if the oxygen demand exceeds the oxygen supply then the DO levels will gradually decline and this presents a serious danger to the koi. Common causes of low DO, apart from high fish densities, are heavy feeding and a dirty pond or filter.
A lot of oxygen can be used in oxidising organic waste and, under certain conditions, this extra demand may be 'the straw that breaks the camel's back'.
Unfortunately, a DO problem often occurs in the early hours of the morning. when we are not there to see its direct effects on the fish, rather than during the day when submerged plants are releasing oxygen from photosynthesis.
Typical clinical signs of low DO are lethargy and a tendency for the fish to gasp at the water surface and congregate around water returns. Many of these signs are the same as for a gill problem so a test for DO has to be made to be conclusive. There are cheap DO test-kits available but it is important to follow the instructions carefully to avoid introducing oxygen into the water sample by agitation or by pouring water into the test phial. If low DO is the problem, i.e. DO is less than 5 to 6 ppm (mg/litre), then additional aeration will help - but it is essential to determine what caused the problem and to take remedial action.