Alan’s Book

Chapter 9 The Living pond

Once you fill your pond with water and introduce your koi, the environment becomes a truly dynamic one which will house a whole host of organisms living and reacting with one another. The koi pond will not contain all of the organisms which reside in a natural pond and waste product removal is largely dealt with by bacteria instead of the host of natural organisms that exist in the natural pond so it is important to realise that a koi pond with a biological filtration system is at best a compromise!

The pond water will contain natural minerals which form the basis of a food chain, plankton and algae. Sunlight entering the pond will provide an energy source to enable them to grow. Algae will begin to grow on the pond walls and floor.

Once the koi are introduced they will excrete ammonia and carbon dioxide; the ammonia will begin a series of bacterial reactions which reduce its toxicity utilizing oxygen at each stage. The organic waste from food and faeces will encourage detritus reducers by way of other bacterial reactions accompanied by the growth of fungi and protozoa of various types.

The carbon dioxide from both koi and bacteria will introduce acids into the pond water as carbonic acid. This will either be mopped up by the buffer system as explained in chapter 4 or lower the pH of the pond water.

If a sample of algae is taken from the pond wall and examined at low power under a microscope a whole new world comes into view. The various types of algae can be distinguished by their cell formation and the larvae of various insects may be seen together with a host of different protozoa.

First of all let us look at some of the basic pond processes which are happening; understanding these processes should help us gain an overall better knowledge of what happens in our ponds.

Although our koi ponds differ considerably from natural ponds, similar processes are occurring in them. For example the water in a newly filled pond often turns green, severely inhibiting the viewing of our koi. The reason for the green water is usually because the newly filled pond has an abundance of plant nutrients dissolved within the water and tiny plants respond to this food source and grow profusely. The green water is caused by single cell algae suspended in the water. If the pond water is left without water changes or top ups the algae will eventually die off as the nutrients within the water are used up; at this time the water will become clear.

If the pH of the water was checked during the “green” water stage, it would be noted that the pH would rise considerably (sometimes over pH 10) during daylight hours, and fall off severely during the hours of darkness reaching a minimum just before dawn. This is due to a process known as photosynthesis where energy from sunlight together with carbon dioxide is used by plants (algae) to grow and multiply. A useful by-product of this natural process is that oxygen is released by the plants into the water and this increases the dissolved oxygen content during the daylight hours. During the night in the absence of sunlight the plants which were using carbon dioxide and producing oxygen now reverse this process and consume oxygen and release carbon dioxide back into the pond water. From the foregoing it follows that carbon dioxide, oxygen and pH levels will change during a daily 24 hour cycle.

A pond containing “green” water will have low levels of oxygen during the hours of darkness as the algae causing the green water will use large amounts of the pond oxygen during this time. It is wise to avoid feeding the koi in the evening when these conditions exist in a pond as the koi’s requirement for oxygen increases substantially after feeding. Under no circumstances should the aeration be turned off at these times.

Our atmosphere is rich in oxygen, containing about twenty percent oxygen, whereas water can only contain much lower levels, from about 15 mg/lt maximum at low temperatures and reducing considerably at the higher end temperatures for survival.

Put another way, there is about 30 times less oxygen in water than in the same volume of air and in water other creatures including bacteria share this essential component of water.

The surface area in contact with the atmosphere (air) provides an interface for air to diffuse into the water; in other words a large surface area will provide larger lung for the whole pond system.

Koi extract oxygen from the water by passing it over their gills, which perform a similar function to the lungs of terrestrial animals. Like terrestrial animals koi also respire carbon dioxide; bacteria also use oxygen and respire carbon dioxide

Carbon dioxide dissolves in water to produce carbonic acid and this tends to lower the pH of the water.

All living things have a range of conditions in which they can survive; bacteria are living things and as such they require certain conditions in which to survive and perform their useful tasks. In a filtered and re-circulated koi pond, one of these conditions is a range of pH between 6.5 and 9 and large quantities of carbon dioxide dissolved in water can lower the pond water pH to 6.5 and lower! If this happens the ammonia removing bacteria are unable to function and remove the extremely toxic ammonia from the pond water with dire results for the pond inhabitants!

Our koi ponds comprise a complex aquatic environment, housing interactions between fish, a host of other planktonic creatures, bacteria, gases, and minerals.

Most pond processes produce carbon dioxide as and end result. Fish and filter respiration are an example of this process which is occurring continually (day and night). As explained above the algae use oxygen and respire carbon dioxide during the hours of darkness. This can severely lower the pH of the water at this time and ponds with low pH levels are vulnerable to filter malfunction (pH crash) at these times. At pH levels low enough to cause filter malfunction the ammonia is much less toxic to koi but combined with low oxygen levels it can severely stress them. PH swings are reduced considerably when the water contains good alkalinity (buffering).

One of the interesting facts about koi keeping is that it touches upon many far ranging subjects such as the material used in distribution pipework for household drinking water to the effect sunlight has upon the oxygen levels of our ponds. It can increase our overall awareness to a whole host of other different but related subjects.

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