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 Good water guide09   (Good water guide)


I wrote the following articles for Koi Carp Magazine.
Therefore they own the copyright but the Editor has given permission for them to be republished here.
Thank you, Karen.

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Aimed primarily at beginners to the hobby, this series of articles will take you step by step through the process of understanding how a good koi pond works.

Part 4:  Improving circulation

In this series, the filter system shown in figure 1 has evolved into a very basic but perfectly adequate solution to the problem of biological filtration and there is also sufficient mechanical filtration to ensure the bio-filter doesnít become clogged with silt and decaying fish faeces.  It is now time to address the defects in the pond itself.

The first thing to notice is that although the addition of a dome for the bottom drain has improved the water circulation a little, there are still ďdead spotsĒ where water that has just returned from the filter system isnít being thoroughly mixed with the water already in the pond.  The pond isnít yet ready for fish but, to see what would be the results of poor water circulation, look at the overall flow and imagine that there were fish in the areas A, B and C.  At the moment, there is nothing to prevent water that has just returned from the biological filter from making its way almost directly to the bottom drain and back to the filter again.

Fish excrete ammonia continuously and so any fish in area B will have its ammonia output taken away as water flows into the bottom drain from the pond return inlet, (so-called because it is the inlet where water returns to the pond). Ammonia couldnít build up in this area and so, in that respect, it would be a healthy place for a fish to be. In areas A and C it wouldnít quite be a case of no mixing whatsoever, but nothing about this pond encourages water to flow through them so they would become almost dead spots in the pond.

Beginner pond + vortex + bio + zones

Figure 1:  Circulation pattern caused by the filter system alone leaves dead spots

A second thing to note is that, although the biological filter is thoroughly aerated for the benefit of the bugs in it, there isnít any provision for aerating the pond for the benefit of the fish it will contain. Aeration will not only provide some much needed oxygen for the fish but, if placed correctly, air stones can help mix the water so that there are no dead spots where ammonia or other pollutants could build up, or where the level of dissolved oxygen could fall.

To clear up some confusion about aeration, when we aerate water we donít just add oxygen.  The atmosphere is composed of several gasses.  It consists of about 78% nitrogen, nearly 21% oxygen, about 0.04% carbon dioxide plus small amounts of other gasses such as argon, neon, helium and minute quantities of about ten other gasses too.  Although it is convenient to talk about oxygenating water by means of air pumps and air stones because oxygen is a very important dissolved gas, in reality, aerating water actually adds nitrogen as well as oxygen.  It also adds a small amount of carbon dioxide and very tiny amounts of all the other gasses too. It obviously isnít important to remember the names of the gasses or their percentages but it can be helpful to remember that, if the level of any dissolved gas in the pond is low, then aerating it will increase the level of that gas in the water.  Or if the level of any dissolved gas is higher than normal, aerating the water will reduce the level of that gas.

There are three cases in a koi pond where this is important. Firstly, if the dissolved oxygen level is low, either due to fish respiration or because oxygen has been removed by a pond treatment, then aeration will increase the dissolved oxygen level. Secondly, if the level of dissolved carbon dioxide has increased due to fish or plant respiration, aerating the pond will reduce the level. Thirdly, and this may seem hard to understand but it is true; if the dissolved oxygen level should become higher than normal due to ozone or hydrogen peroxide being used, aerating the pond will actually reduce the oxygen level.

It would be going off at a tangent, at this point, to explain why ozone or hydrogen peroxide might be used.  Those who use them know why they do so, but it is worth while considering whether, for instance, adding hydrogen peroxide at the same time as aerating a pond would produce the desired effect. There are arguments for and against this course of action because hydrogen peroxide will be trying to increase dissolved oxygen but, depending on actual circumstances, at the same time, aeration may also be trying to remove the extra oxygen just added.

How does aerating water actually work?
This is a simple enough question but, from answers given on koi forums, the way bubbles aerate water isnít always well understood so let us begin with some fundamentals.  If water with no dissolved oxygen in it is exposed to the air, the molecules of oxygen in the air above it create a pressure on the surface. There are no molecules of oxygen already in the water to push back against them so the opposite of a tug of war takes place.  In this case there is a ďpush of warĒ.  One by one, molecules of oxygen are pushed into the water. As the number of molecules in the water increases, they start pushing back. Eventually, the push from the dissolved molecules, exactly balances the push from the atmosphere.  The push of war reaches a stalemate and the whole process slows to a halt.  This also happens with all the other atmospheric gasses but since there are only two gasses that we are particularly interested in; oxygen and carbon dioxide, the others can be ignored.

A similar push of war will also take place if the water initially had no carbon dioxide in it. Molecules of CO2 above the surface will push down and, one by one, CO2 molecules will be pushed into the water until the push from inside exactly balances the push from the atmosphere above, just as with the oxygen. The water will now contain some oxygen and a trace of CO2.

If a fish is put into the water, its gills will take up some of the dissolved oxygen molecules and replace them with some CO2.  Two things now happen simultaneously.  With some of the dissolved oxygen having been removed from the water, there is less push from it to balance the oxygen pushing from above so more oxygen is pushed in to take its place.

At the same time, the extra CO2 in the water is now pushing back at the atmosphere with a little extra force, resulting in the excess CO2 being pushed out. With a few fish in a pond that has a large surface area, the process of oxygen being pushed into the water to replace what the fish have used, while at the same time, the waste CO2 is pushing its way out, will keep dissolved oxygen and CO2 at normal levels.

Increasing the pondís surface area
The speed at which oxygen can be pushed into a pond, and excess CO2 can be pushed out is only limited by its surface area.  This means that a pondís surface area is the limiting factor on the number of fish that can be comfortably accommodated in it.  To increase the number of fish that can be held, the surface area somehow has to be effectively increased and this is where aeration can help.

When air is introduced into the pond via an air stone, it rises in a stream of bubbles.  Depending on the size of the bubbles, there can be a huge surface area between the surfaces of all the individual bubbles and the water surrounding them.  Consider the case of a small pond that has a surface area of 5 m2; if 1 litre of air is pumped into it through an air stone which will turn that 1 litre of air into 1,000 bubbles then the total surface area of all those bubbles adds up to 4.9 m2.  In other words, 1 litre of air in the form of 1,000 bubbles effectively almost doubles the surface area of that pond.

If the air stone produced a much finer stream of bubbles, say 10,000, then the total surface area of the bubbles is more than doubled to 10.4 m2.  This much finer stream will have effectively tripled the surface area of the pond.  And that is what happens when just 1 litre of air is bubbled into a pond. A 50 litre per minute or higher output pump has a much greater effect on the effective surface area.  Another effect that causes an increase in surface area happens when the rising current of bubbles reaches the surface. The bubbles donít burst immediately, they float across the water surface for a few seconds and the thin film of water that forms the bubble is exposed to air both from the inside and the outside. Also the disturbance on the surface causes ripples that further increases the total surface area of the pond.  With suitably placed air stones fed from a good sized air pump, a small pond could have a surface area equivalent to a small lake.

Beginner single air-stone

Figure 2:  Single air stone circulation

Aeration causes currents in water
This is stating the obvious but one of the defects in the pond design so far is that water circulation in it is poor, and the other is that it isnít aerated so as to quickly replenish oxygen and remove carbon dioxide. It would be well to see how air stones could solve both problems at the same time.

There are no hard and fast rules due the wide range of different size and shapes of ponds but there is a good general principle that should be considered.  Instead of putting small air stones, equally spaced around the pond as is often seen, consider what would happen if all the air was introduced in one place close to a wall as in figure 2.  As the bubbles rise towards the surface, there will be the expected upward current of water.

As this current reaches the surface, it has to spread out away from the wall.  At the bottom of the pond, water will be drawn towards the air stone to replace water that is moving upwards. The effect of these two currents will cause a downward current at the wall opposite to the air-stones. This will form a slowly circulating current of water taking aerated water all across the width of the pond, down the opposite side and back across the bottom which will ensure that there are no dead spots in these areas. This will work for most average sized ponds but if there are concerns that any particular pond is too wide and that there wonít be sufficient aeration on the far side, it is a simple matter to position air stones against the walls directly opposite each other and get the effect in figure 3.

Beginner double air-stone

Figure 3:  Double air stone circulation

Although, for simplicity, a single air stone is shown in each diagram, in practise, a group of air stones or a ceramic plate air diffuser is needed in order to introduce enough air to create a sufficiently powerful upwelling of water to start this cycle.

Beginner return central

Figure 4:  Pond circulation with a central return

Beginner return offset

Figure 5:  Pond circulation with an offset return

The plan view, in figure 4, shows water flowing from the return inlet directly towards the bottom drain. Since the simplest solutions are usually best, when building the pond, some thought should be given as to how the returning water will behave.

In the vortex design, as described last month, water enters at the bottom and is offset from the centre causing it to take a slow helical path before it can leave at the top.

If the pond return inlet is offset from the centre and is positioned near the top, the opposite will happen.  There will be a slow circulating current all around the pond and travelling from the surface to the bottom drain.

Coupled with currents caused by the air diffusers, our pond now has good circulation and with a few more improvements will be ready to be stocked with some fish.


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