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Pond Construction

Aerating the pond

The next step in pond building is to consider the most effective way to aerate a koi pond
Chris Neaves wrote a very comprehensive article about the oxygen levels in ponds as well as the oxygen levels that koi need to survive in a pond. The article is called Bursting Bubbles. Some interesting comments made in the article may be repeated throughout this particular subject to support the importance of aeration in koi ponds.

The most important aspect in a healthy koi pond is oxygen and therefore we need to consider ways to increase and maintain a high level of oxygen in the water. Unfortunately, competition for oxygen in the water environment is rife. In practice, the instant that oxygen is diffused into water, all the organisms in the water will start competing for this scarce resource. Oxygen should therefore be encouraged back into the water on a continuous basis.

 Algae produce oxygen during the day but remove it at night. Studies have found that in ponds with relatively high stocking densities and blooms of plankton algae, the algae removed more oxygen from the water than the fish during the course of the night.
Fish need oxygen to convert food into growth and energy. In warmer water this is happening all the time. Therefore, the oxygen levels have to be high all the time.
Many factors will influence the solubility of oxygen into water. The most important is atmospheric pressure, altitude, temperature and salinity.

The bio filter consumes considerable amounts of oxygen converting ammonia to less toxic substances. The more mature the filter the greater the biomass that needs oxygen.
Decomposing material consumes oxygen.

Millions of bacteria exist in every 1cc of pond water and they all utilise oxygen.

The total dissolved organics will determine the water’s ability to carry oxygen.

Some chemicals when added to pond water acts as oxygen scavengers.

Well designed aeration and circulation encourages other harmful gases and excess nitrogen and carbon dioxide to escape to the atmosphere.

With the exception where some complicated devices are used, oxygen can only enter the water by diffusion between molecules of water in contact with atmospheric air. In other words, the ideal will be to bring all the water's molecules into contact with the atmosphere at some time or the other. Although the surface area of every air bubble created in the water will assist in gas exchange, it is also generally accepted that most diffusion of oxygen into the water and the release of unwanted gasses into the atmosphere takes place at the surface of the water because of the large contact area. In order to achieve this, the pumping system must run 24 hours a day and the turnover rates should be as fast as possible.

The question that the pond owner needs to ask is whether a high turnover rate is good enough?
In view of the various factors mentioned above the answer is not an easy one and will differ from pond to pond, depending largely on the filtration system, depth of the pond, surface area of the water, chemical composition of the water, temperature as well as the way the water returns to the pond. I will give two examples.

In the first case a pond is equipped with a nexus, from there to a baki shower. Even if the filtered water returns to the pond via mid-water returns, the oxygen content of the water will be very close to saturation level, provided the circulation and mixing in the pond is sufficient.

In the second case (like the majority ponds in South Africa) a pond is equipped with a settlement chamber/vortex, some static filtration and then sand filter and closed pressurised filters. Water is returned via a water feature and some mid-water returns. The water returning via mid-water returns will be virtually devoid of oxygen. Even if the turnover rate is many times per hour, the oxygen content of the water will not even be close to saturation level.

In essence the pond depth and shape, water temperature, water quality as well as the filter design will determine the need for extra aeration.

There are various ways, all with different success rates that can be employed to aerate ponds. In fact there are also so many permutations that it will be impossible to take all of them into consideration. I will therefore focus on a few well known techniques, but in order to demonstrate the various ways of aerating, the assumption must be made that no other ways of circulation exist in the pond. It is up to the pond owner to decide what technique will/can be employed in a specific pond. Please note that all techniques of oxygenating the pond will raise the oxygen level to saturation point in a pond without any koi. As soon as you ad koi to the pond, the oxygen consuming cycle stars. It is therefore the speed that oxygen is replaced that matter and it will determine the aeration technique that will be employed by the keeper.

The waterfall is one of the most efficient ways of aerating a pond. The success of a waterfall will depend on the volume of water going through the waterfall, height and width of the waterfall and of course the agitation of the stream as it cascades over a few layers or pebbles to bring as much of the water molecules into contact with air. In a medium to large pond, a waterfall will aerate the immediate area where the water returns and therefore good circulation is essential to distribute the oxygenated water evenly trough the pond. As the water splashes back into the pond, it also creates enough surface movement to increase the surface area exposed to the atmosphere.


Multiple small airstones

Multiple airstones are frequently used in ponds and are moderately successful. This way of aeration do bring some water to the surface and creates ripples on the water that will increase the surface area of the water exposed to the atmosphere. It is doubtful if the smaller airstones will “lift” sufficient volume of water from the bottom of the pond to keep the water sufficiently aerated. When viewed from the top it can be seen that the volume of water brought from below is insufficient as is the agitation of the surface. 






Large diffuser disc on Bottom drain

As can be seen in the illustration, a disc shaped diffuser will create a large column of air with the accompanying huge lift. It will inevitably also draw substantial amount of water from the bottom of the pond and sides of the column and forces it to the surface. In this process it will create currents as depicted below and over time will expose a lot of water molecules to the atmosphere. This technique works well in a deep pond with a small surface area. 




“Flooming” is the term used for the effect created by a small submersible pump placed on the bottom of the pond which forces a jet of water towards the surface. As the jet of water continues towards the surface of the pond, it has to overcome the natural resistance that exists in the pond’s water body, causing the upward energy to disperse into the surrounding water. This creates a mushroom effect and the column of water that reaches the surface has a much larger diameter than the original jet created by the small pump. Flooming is a very effective way of increasing the surface area of a pond for gas exchange, but tends to recirculate the top half of the pond’s water. The limiting factor to this technique is that the pump cannot move more water from the bottom of the pond than the maximum pumping capacity of the submersible pump.




Submersible pump with extension and elbow at surface

This technique creates a lot of agitation on the surface of the pond. This agitation when viewed from the top tends to be V-shaped and only part of the surface are agitated sufficiently. Again the limiting factor to this technique is that the pump cannot move more water from the bottom of the pond than the maximum pumping capacity of the submersible pump and has to be assisted by other ways of circulation in the pond.




Aeration hose

The aeration hose has actually made it possible for pond owners to create an air curtain that can be shaped in various ways to fit any specific pond, even large circles. It also has the ability to generate micro bubbles that must be viewed as the best way of aeration with an air pump. This rubber hose with micro fine holes should be anchored in the pond because of it’s tendency to float. The best way to keep the hose in place is to push a 10 mm stainless steel rod through the pipe to reduce the buoyancy
Because of the length of the hose and the micro bubbles, technique will deliver bubbles over a wide area with the accompanying massive lift of water and the creation of a wide, even current where over time the most water will come into contact with the atmosphere.   



Water fountain

A water fountain are based on the same principles as flooming with exception that it has an extended pipe with a spray nozzle on top that forces water in the form of a spray fountain  into the air that breaks up into hundreds of individual drops before it splashes back into the pond. The oxygen saturated drops creates more agitation on the surface for further gas exchange.
Again the limiting factor to this technique is that the pump cannot move more water from the bottom of the pond than the maximum pumping capacity of the submersible pump and has to be assisted by other ways of circulation in the pond.



The aspirator is nothing more than a high velocity jet of water that is aimed at the surface of the pond at a 60 degrees angle, drawing air into the water and creating tremendous disturbance of the surface area and penetrating deep into the water body of the pond. In a study conducted in Australia it was found that in mud ponds, this technique is the second best to employ. The study rated the “paddlewheel” the best technique.



The above illustrates the various ways to aerate your pond. The techniques can all be implemented because they are all acceptable ways of aeration. Some techniques are just more effective than others. The effectiveness will really depend on the specific pond, because the following variables will influence the decision:

Pond shape, surface area, depth, filter system, turnover rate, water returns, Stocking density, chemical composition of the water, atmospheric pressure and power consumption.

If one takes into consideration the effectiveness and powerconsumption, the best bet will be the aeration hose.


Last Updated on Monday, 18 April 2011 20:39