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Koi pond aeration is important to supplement dissolved oxygen levels

Koi pond aeration ... Before we even start to think about the oxygen requirements of our koi, our pond is already full of oxygen consuming organisms.

Not only do these organisms demand oxygen, they must receive it insufficiently high concentrations so that the function and health of the koi pond is maintained. Diverse populations of micro-organisms such as autotrophic and heterotrophic bacteria, fungi and microscopic invertebrates rely on a supply of oxygen, just as a burning fire does to keep it functioning effectively.

Deny them oxygen, and the water quality will deteriorate; add extra and the pond will roar into life, just as a bonfire does in a gust of wind.

Oxygen is everything in a pond

Without oxygen, filter bacteria cannot oxidise (and detoxify) ammonia into nitrite and then into nitrate. If a pond or filter are allowed to become oxygen deficient for any period of time, anaerobic conditions will prevail causing unstable water conditions and the tell-tale bad-egg smell.

So how do we best use those devices that are available to us as koi keepers to add extra oxygen into our demanding ponds? How do they work, and how can we measure how efficient and effective we are at adding extra aeration?

Dissolving oxygen into pond water.

The physical nature of water puts limitations on how much oxygen we can expect our pond water to hold. It also sets our goals for what we should expect to achieve under different pond conditions. In the air we breathe, there is approximately 21% oxygen. In any natural water body, we will typically only find 1% dissolved oxygen, and usually less than that. So minute are the concentrations of dissolved oxygen in pond water, that their levels are measured in milligrams per litre or parts per million (ppm). For example 3 milligrams of oxygen dissolved in a litre of water equates to 3 ppm.

A number of physical factors affect the levels of DO that can be achieved in a given pond.

Temperature: Oxygen behaves in a surprising way (compared to other solutes, such as salt) in that as the water temperature rises, its ability to dissolve oxygen reduces. Compare this to salt (which dissolves better in warm water) and you can start to appreciate the unusual relationship water has with oxygen.

Other solutes.

The presence of other solutes in the pond will also have a negative effect on achievable D O levels. This would include salt, as well as liquid pond treatments. All the more reason to aerate vigorously under treatment conditions.

Saturation levels.

These physical factors will determine how much DO your pond water body will be able to hold, setting the levels at which pond water will become saturated with oxygen.

At lower temperatures

Water will dissolve greater levels of oxygen, making it more difficult for your aeration systems to completely saturate your water. Conversely, at high water temperatures, saturation can be achieved more easily as water can only hold reduced levels of DO. By taking into account the saturation levels of pond water under different conditions, it is then possible for you to measure the effectiveness of your aeration system.

  • For example, at 10 degrees C, the water's approximate saturation level will be 11.5 milligrams per litre where at 20 degrees C it will be 9 milligrams per litre. You can quickly measure the saturation level of your own pond by using the graph below (see figure 1), and substituting in your pond's temperature and oxygen readings.
  • Draw a straight line between your two readings on the two horizontal lines and the saturation level is the value where the line intercepts the saturation scale. The closer your saturation figure is to 100%, the better your system is performing. Could your aeration system be performing better?

How much DO do I need?

Using the illustration of bellows on a fire, it safe to say that the higher the DO the better the function and quality of life in the pond. Fortunately, koi being members of the Carp family, compared to some other fish species, require relatively low levels of DO (6 milligrams per litre). From Figure 1, you can see that even at the highest UK temperatures we are looking to achieve approximately 70 per cent saturation of oxygen. But if you can improve on that , the better all the other pond organisms will function. You can also see from Figure 1 why trout (which generally require a high DO of 11 ppm) must be kept in cool flowing water.

How do aeration systems increase the DO?

How an air pump works.

The most common form of air pump found around a koi pond is one that uses a rubber diaphragm. The pump takes in atmospheric air, compresses it and delivers that air under pressure to an aeration device (usually a diffuser). The pump itself consists of an electromagnet and a permanent magnet, attached in some way to a rubber diaphragm. As the charge of the electro magnet changes many times each second, the permanent magnet (which is free to move) is attracted and repelled the same number of times each second. This causes the diaphragm to move rapidly to and fro, causing the air to flow into and out of the diaphragm. The suction, compression and discharge is achieved through the use of one way valves that only open on either suction or discharge.

Pumps are rated on three measurable features. Their power consumption (measured in Watts), their flow rate (or volume, expressed as litres per minute) and the pressure to which the air can be pumped. An air pump will have a performance curve (Fig 2) that shows the relationship between the pump's output (in litres per minute) and the pressure (or depth of pond) to which the air can be delivered. You will notice that the deeper the air has to be delivered, the lower the flow rate becomes. This will also show whether a particular pump is strong enough to deliver air to the bottom of your pond. The air is usually delivered through a manifold to allow numerous diffusers to be used.

Different methods of aeration.

  • Moving/flowing water
  • Venturi
  • Diffused aeration

Moving/ flowing water.

Just as sugar dissolves far better in a cup of coffee when it is stirred, so too oxygen dissolves better in water that is more highly energised (eg flowing, falling and cascading). In a natural environment, more energetic water courses such as Highland streams are higher in DO than the lowland rivers, and besides the oxygen that is released by aquatic plants, water movement is the only method by which water can naturally become oxygenated. In a koi pond, a waterfall can be an effective means of aerating the water but in an intensively stocked pond, can lead to limitations. Oxygenated water that enters the pond at the waterfall will rarely sink to lower layers of water, being effective only at the surface. Furthermore, the physical work required to move water to oxygenate is significantly less efficient than moving air, especially when it only affects the upper layers of a pond. What more efficient methods can be used to oxygenate pond water?

Venturi.

This method is most easily installed through the wall of a pond from a final filter chamber. A venturi is a cylindrical hollow tube with a unique shape through which water is pumped. The unique shape is often referred to as an hour glass by which the diameter of the tube entrance tapers down to a smaller diameter, and finally returns to its original diameter. The effect that the internal taper has, causes the velocity of the pumped water to increase. At the minimum diameter (or throat) of the venturi, the pressure drops only to eventually recover by the end of the venturi tube. If the pressure loss is sufficient at the throat, then a small hole at the throat connected to atmospheric air to be sucked into the returning pumped water, literally injecting air into the pumped returned water.

A venturi can look quite impressive as an aeration device, but in fact, compared to air pumped into the pond water directly, are relatively inefficient aeration devices. They are energy inefficient in that they involve the pumping of water (rather than air) and the bubble size is very large. This means (as we will see later) that the rates of oxygen diffusion will be reduced. The large bubble size also means that only the upper layers of water receive the full benefit of the freshly oxygenated water.

Diffused aeration.

There are many different types of diffusers available to the koi keeper, ranging from the basic (and disposable) airstone to other larger (and permanent) self-cleaning diffusers. Typical diffused aeration provides a constant stream of air bubbles, rising through the water column. A fine bubble size is essential to provide the maximum surface area for efficient oxygen transfer.

For example, if a bubble size of 1/4" is reduced to a fine bubble a quarter of that size (1/16") the surface area is increased by a huge factor of 16! Even if some air diffusers look impressive by the sheer volume of large bubbles they are relatively inefficient as they provide a relatively small surface area for gas exchange and most of the pump energy is used to blow air back into the atmosphere. Contact time between air and water as bubbles rise through the water column is also important. As water depth increases, contact time also increases combined with the increased head of pressure providing better physical conditions for dissolving oxygen into water.

Furthermore, if fine bubbles are generated by a suitable diffuser from depth, their large surface area creates a drag against the water, reducing vertical velocity increasing contact time further. An added benefit of this "drag" experienced by a rising column of fine bubbles is the mixing and circulation of significant volumes of water. This mixing action enhances water quality by making the pond a well-aerated, homogenous and stable environment. Fine bubble aeration also reduces surface agitation, permitting continued and undisturbed viewing of your koi.

Traditional air diffusers (airstones) are perhaps now regarded as inefficient because as the volume of air supplied is increased to increase aeration, the bubble size increases reducing gas exchange rates. They are also prone to clogging and blocking up, reducing airflow considerably.

A relatively new and unique flexible rubber diffuser (called an airdome) is now available which has been specifically developed to satisfy all the necessary criteria for optimum gas exchange. In addition the flexible nature of the unit has self-cleaning properties that ensure optimum efficiency during long-term use. This can be fitted onto a bottom drain where it aerates the pond from the bottom, giving enhanced aeration in addition to mixing of the whole water column, enhancing the removal of waste towards the bottom drain.

In conclusion, the levels of DO achievable in a koi pond are determined by natural physical conditions, particularly water temperature. We have various options when choosing an aeration device, with some methods proving more efficient and effective than others. The most efficient method by far is using diffused aeration, but the overall performance is still determined by the careful selection of air pump and diffuser.

7 things you should know about koi pond aeration.

  • Unlike other substances that dissolve in water, oxygen is LESS soluble in warm water.

  • Even though koi only require 6mg/l DO, both they andthe other pond organisms will benefit from higher DO levels.

  • Diffused oxygen is the most efficient method of aeration

  • Small bubbles are far more effective at aerating than larger bubbles.

  • Venturis and water falls may look impressive as aerating devices, butin fact they generally only aerate upper layers of pond water.

  • The addition of airstones to a biochamber will enhance its performance. Do not aerate a mechanical chamber as this will adversely affect settlement.

  • Koi require more aeration in warmer weather and during pond treatments as both these features reduce your pond water's ability to hold oxygen.

water pond oxygenate aeration air koi diffusers bubble dissolve saturation pump temperature litre venturi "aeration device"

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