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Dissolved Oxygen




Koi Pond Aeration is dependent on the Ponds Dissolved oxygen. Dissolved oxygen (DO) refers to oxygen gas that is dissolved in water. Fish "breathe" oxygen just as land animals do. However, fish are able to absorb oxygen directly from the water into their bloodstream using gills, whereas land animals use lungs to absorb oxygen from the atmosphere.

There are three main sources of oxygen in ponds:

  • Direct diffusion from the atmosphere.
  • Wind and wave action.
  • Photosynthesis. Of these, photosynthesis by aquatic plants and phytoplankton is the most important.

Photosynthesis: Photosynthesis is a process whereby algae and aquatic plants use carbon dioxide, water, and sunlight to make their own food. Oxygen is a by-product of this activity. Therefore, as long as photosynthesis is taking place, oxygen is continuously being released into the water.

Oxygen, derived from photosynthesis, is produced during the day when sunlight shines on the plants in the water. Oxygen levels drop at night because of respiration by plants and animals, including fish. These predictable changes in DO that occur every 24 hours are called the diurnal oxygen cycle.

Oxygen Depletion

Oxygen depletion refers to low levels of DO and may result in fish mortality. Until recently, the recommended minimum concentration was 6mgl but this has been raised to 7mgl for healthy growth, tissue repair and reproduction.

Mortality usually occurs at concentrations less than 3 mg/L. The number of fish that die during an oxygen depletion event is determined by how low the DO gets and how long it stays down.

Don’t assume that if oxygen levels in the pond drop, Koi will be seen gasping at the surface, in reality, at this stage the Koi are minutes from dying. The first sign of low oxygen is when Koi, who should be actively swimming and feeding, behave as though it is winter, lying on the pond bottom, neither feeding nor swimming. You should immediately increase aeration to the pond, in an emergency, a pump can be placed in the pond and allowed to splash water over the pond surface. In the long term, consider installing an air pump.

More fish die from a lack of oxygen than any other cause. Oxygen is the first miting factor in water quality. Ammonia and nitrite take days to reach crisis levels. Oxygen can become critical in a few hours.

In garden ponds, we typically see oxygen problems only during the summer because when the water is warm all those things which consume oxygen speed up increasing their consumption of oxygen. Your fish may be happy and healthy at 68° F (20° C) and suffering from low oxygen stress at 86° F (30° C) in the same pond.

The Following All Use Oxygen:

  • Koi & Pond Fish
  • Plant Respiration
  • Chemical Decomposition of Waste Matter
  • Bacterial Decomposition of Waste Matter

What Causes Oxygen Depletion?

Oxygen depletion occurs when oxygen consumption exceeds oxygen production. Increases in oxygen consumption can be caused by an over-abundance of aquatic plants or algae in the ecosystem, "turnover" of a body of water (see Stratification/Pond Turnover section), increased organic waste entering the water (i.e., manure from feedlots, septic tank waste water, and excess fish feed), death and decay of organic matter (i.e., plant or algae die-offs), or by certain chemicals (i.e., formalin) that remove oxygen directly from the water column.

When your pond is low in dissolved oxygen you are stressing your fish, which makes them much more vulnerable to disease, parasites and infection. Their activity level will be reduced as well as their growth rate. Low oxygen levels will lower the oxidation/reduction potential (ORP), favor growth of disease causing pathogens and disrupt the function of your biofilter.

Most Koi keepers know they need aeration in their water, but assume that the waterfall, air diffuser or fountain is providing enough oxygen for the pond. It is important to know if you have an oxygen problem long before your fish start dying. You need to either monitor it by periodically measuring it when you expect it to be low or, select and use an aerator large enough to handle the summer time conditions.

Oxygen is usually highest in the afternoon and lowest around 3-4 am and can be reduced lower at night by plants in the pond. Koi need at least 6mg per liter of oxygen for health. This is easy with cold water with a saturation level of 14 mg per liter but warm 80° F water in summer saturation level is 8gm per liter and this is when Koi can have problems. Also, when you use pond treatments like potassium permanganate or formalin, oxygen is used up by the treatment and fish can be found gasping at the surface. A quick solution to this problem is to spray hydrogen peroxide into the water to oxygenate the water or to place an air stone into the water to aerate it.

It is recommended to add fresh water equal to 10% minimum of your pond weekly.

Decomposing aquatic plants and/or algae can result in the loss of oxygen in a waterbody, it works like this: once the plants or algae die, a feeding frenzy is often triggered within the detrital aquatic community microbes and/or insects that feed on rotting vegetation and debris), as bacteria begin to break down or “decompose” the dead vegetation. The increased activity can result in a loss of oxygen because these organisms are working harder and therefore using more oxygen. If there is a large amount of dead vegetation or algae, such activity can result in a severe loss of dissolved oxygen and, consequently, a fish kill. Lakes or ponds with heavy populations of aquatic plants or algae are more susceptible to this type of event and can result in large numbers of dead fish. That is why, when using chemicals (algicides or herbicides) to remove algae or aquatic weeds from a pond you need to add more oxygen with aeration pumps.


Weather Conditions

Warm water is much less capable of holding oxygen gas in solution than cool water. For example, water that is 90° F can only hold 7.4 mg/L DO at saturation, whereas water that is 45° F can hold 11.9 mg/L DO at saturation. This physical phenomenon puts the fish in double jeopardy because at high water temperatures their metabolic rates increase, hence their physiologic demand for oxygen increases.

Muggy, overcast summer days often precipitate oxygen depletions. During cloudy weather, the intensity of light reaching surface waters is greatly diminished, resulting in a marked decrease in oxygen production from photosynthesis. Oxygen consumption, however, remains unchanged. This results in a net loss of oxygen over each 24-hour period. This loss of oxygen from decreased production is confounded by still, muggy, humid weather common on overcast summer days. Oxygen transfer (from the atmosphere into the water) is minimal because there is little or no wind/wave action. The net result over a period of several days is oxygen depletion and, often, fish kills.

Heavy thunderstorms can also have an adverse effect on oxygen levels, especially after extended periods of dry weather or during hot weather. If conditions have been dry for a long time, heavy rains tend to wash large amounts of organic matter such as dried leaves, grasses, into the pond. As bacterial organisms begin to decompose the new material, oxygen is used at a faster rate than normal. This can be a real problem during hot weather as there is less oxygen in the water.


How to Determine If Low DO Is the Cause of a Fish Kill:

  • All fish die at approximately the same time (often during the night or in the pre-dawn hours).
  • Large fish may be affected more than small fish.
  • Koi fish may be seen at the surface "gasping" for oxygen (this is called "piping").
  • Some species may die with their back arched, gills flared and mouth open. This is most commonly seen in hybrid striped bass and, occasionally, in catfish.
  • The weather immediately prior to the fish kill may have been hot, still and overcast. A severe thunderstorm may have occurred immediately prior to the fish kill.
  • An oxygen depletion event severe enough to result in significant fish mortality is often observed in water with heavy populations of algae or aquatic plants.

The most important thing to do if fish are dying from low DO is to turn on an aerator. To avoid fish kills keep and air pump running especially when treating your pond with algicides or herbicides.





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Dissolved Oxygen (Heat Stress)

Goldfish, Crucian carp (Carassius), and Koi are three species that switch to anaerobic pathways in times of low oxygen. However, prolonged exposure to these conditions can risk an acidosis of their tissues secondary to lactic acid buildup.

Shade can be provided to fish ponds with either a shade cloth or non-pressure treated wood.

Fish exhibiting signs of heat stress will be congregating at the surface of the pond, near the waterfalls, or any other area where oxygen levels might be a little higher, or where there is shade. The thermometer should aid you in making the diagnosis. Warmer water does not carry oxygen as well as cooler water. At different temperatures, the saturation point varies by as much as 50%, more or less.

In addition to the fact that warm water does not carry as much oxygen, it is know that fish demand more oxygen in warmer water. So at the very same time the water carries less, the fish are wanting more!













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