Aeration and Oxygen in Tropical Koi Ponds


The key factor that controls the Dissolved Oxygen ( D.O) concentration in ponds  is the solubility of Oxygen in water.   Oxygen has a hard time dissolving into water and the amount that can be dissolved is heavily influenced by temperature.

The maximum amount of oxygen that water can hold, at any given temperature, is called the saturation point. It's a bit like dissolving sugar in a glass of water…at a certain point no more will dissolve because the water is “saturated”.

The saturation point of cool water is almost double that of warm water meaning cold water can hold twice as much Oxygen as warm water. Naturally the maximum amount of oxygen that can dissolve into warm Caribbean ponds is therefore dramatically less than in cooler climates.



What makes things worse is that its VERY difficult to even reach this saturation point and the warmer the pond gets the harder it is to get Oxygen to dissolve.

Oxygen dissolves into water at the Air / Water interface - this can be the surface of the pond or the surface of an air bubble as it rises. The larger the contact area and the longer the contact time between air and water the better.

This is why an air stone that produces fine bubbles is much more efficient at oxygenation as lots of little bubbles have a much greater surface area than a few big bubbles. It also explains why deep ponds with a small surface area often suffer even more from low oxygen levels.


There are also other points to consider:

• Fish, Bacteria, plants and algae as well as biological filters, decaying matter in the pond like leaves or undigested food residue all use up Oxygen. The warmer it gets the more Oxygen is used. Even the fish themselves need more Oxygen in warmer water - Imagine a Koi in a cold pond, it will be fairly inactive and will have a slow metabolism, meaning it will use less Oxygen. The same fish in warm water will be more active and have a faster metabolic rate and so will require more Oxygen. So whilst the Koi demand more oxygen in warm conditions there is actually much less available.

• If the oxygen level is low, as in most tropical ponds, Koi will be chronically stressed. The fish often show no visible signs of this and will not be gasping at the surface, but their entire physiology will be compromised. This results in reduced growth rates, poor digestion and increased susceptibility to disease. If Oxygen levels are already low it may only take a small change to tip the balance.

• When Oxygen levels fall too low, fish quickly die and large fish usually die first - this can literally take minutes. As the fish die they immediately start to decompose which uses more oxygen, resulting in a domino effect often killing every fish in the pond. This happens much faster in warm water ponds. It is a fact that more Koi are killed by lack of Oxygen than by any other cause!

• Oxygen levels are also dependent on air pressure. It stands to reason that the higher the air pressure pushing on the water surface the more oxygen will dissolve into the water. A storm or low-pressure weather system passing over will mean less oxygen in a pond and can lead to large fish kills.

• Pond water needs to be mixed properly, otherwise the deeper areas often become oxygen deficient. Vertical mixing is critical and the best way to get this is with an air pump.

Low oxygen levels also reduce water quality and clarity as well encouraging disease causing bacteria and parasites. The beneficial bacteria that live in the filter need plenty of oxygen to function efficiently, so low oxygen levels reduces filtration efficiency.

It is also worth noting that oxygen is not the only gas we should be concerned with, CO2 the waste product of metabolism and other metabolic gasses are also exchanged by aeration. Aeration allows oxygen to dissolve in to the pond water whilst allowing unwanted gasses to escape.
The Japanese Koi farmers appreciate the importance of strong aeration and I have never seen a Japanese holding ponds without huge amounts of aeration.



How do you increase Oxygen levels in your pond…

1. Air Pump:
The absolute best way is to fit a powerful air pump with a proper diffuser or air stones. Be careful of cheap piston type pumps as these are often noisy, unreliable and some contain oil, which can pollute the pond.
Aquarium air pumps are far too small for proper pond aeration. To maintain proper dissolved oxygen levels in tropical ponds I recommend adding 30 Liters of air per minute for ever 1000 U.S gallons of pond water pumped through air stones or diffusers. Our air pumps are specially supplied by the manufacturer for use on the Barbadian power supply.

2. Hygiene:
Keep your pond and filter well maintained and clean. Decomposing leaves, fish waste and sludge in the pond and filter all use lots of oxygen.

3. Submerged Plants:
Whilst aquatic plants produce beneficial oxygen during the daytime they use it up at night, so be cautious of having too many submerged plants if your pond contains a lot of fish. Remember dead plant leaves will also use oxygen as they decompose.

4. Use a proper Koi Food:
Do not feed Koi with food intended for other species!
This includes dog food and Tilapia pellets. The Koi cannot properly digest these and so a lot of the food is released as undigested waste, this decomposes using valuable oxygen.
Its also worth noting that these feeds also have some other nasty long term side affects, including liver and kidney failure, poor growth, malnutrition, vitamin deficiency, obesity and reduction of water quality. Better quality Koi foods, such as the Kokugyo brand, are highly digestible, meaning less waste and therefore higher Oxygen levels.

5. Encourage surface water movement and mixing
This is useful to increase oxygen levels throughout the water column. Waterfalls, Showers, water jets, venturis and fountains all help but should not be the sole source of oxygenation.

This is a very brief overview of what is a complex subject. In reality there are a lot of other factors that come into play such as partial gas pressures, altitude and the complex biological and chemical systems within a pond.

The main thing to take away is that warm water ponds are almost always oxygen deficient and the hobbyist will see real benefits from increasing aeration.


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