Alan’s Book

Chapter 7 Comparisons between the Natural Pond and our Koi Ponds

The availability of food in a natural pond is directly linked to the “productivity factor” of the pond water, that is, to the nutrients available in the water. Soft water contains fewer nutrients than hard water so it follows that ponds with hard water are more productive due to the higher level of nutrients that they contain which provide food to the life forms in the lower levels of the food chain.

In a natural pond a balance exists between the pond nutrients and the stocking levels of fish. The food chain begins, and is limited by, the amount of nutrients (plant food) available in the water.

These nutrients consist of minerals which include nitrates and phosphates that allow plant life to grow. In soft waters nutrients can be “locked up” in the mud and are not available for use due to low pH and the shortage of calcium and other minerals.

Various invertebrates such as water louse and shrimp use decaying plants as a food source. These creatures form, together with the plants, a source of food for the fish. As a matter of interest, the smaller plant/insect feeding fish are used as food by carnivorous fish higher up the food chain.

So a natural balance occurs which is limited to the closed environment of the pond, the nutrient value of it, and the pH.

In a natural pond, the way to increase the quantity and growth of the fish, is to add fertilizers/lime to the pond, water thereby increasing the nutrients>plants>insects>fish. The food chain is complex but in simple terms dead plants and fish, together with fish faecal material, release nutrients back to the water and these nutrients are then re-used by the plants to fish process, and so the cycle continues.


In koi ponds we aim for clear water, often using UV units to control the single cell algae (plant) that produces green water, so that the fish can be seen and appreciated, and we avoid having other plants in our ponds for a variety of reasons.

We immediately come into conflict with nature in that we are removing the plants. These plants would normally provide food, shelter, and a valuable contribution to the removal of waste products within the pond system.

Because plants directly utilize fish waste products (ammonia) quite efficiently, there is more work for the nitrifying bacteria to do in filtered ponds, that is, convert the ammonia through to the plant food as nitrates. As we all know from previous chapters, ammonia is very toxic to fish so it is essential that a balance exist, which uses the ammonia quickly before it accumulates to toxic levels in a pond environment.

From the above we can see that our ponds are seriously out of balance with nature already.

How do we put this right?

Having removed an important part of the natural food chain we can buy fish food and feed the fish, Sounds fine – but how much must we feed our fish?

The food is no longer limited by what the pond can produce from plant food (dissolved minerals) at various times of the year and dependant on the amount of sunlight available or the temperature of the water: it is now controlled by the Koi keeper.

Because food is no longer a limiting factor to the amount of fish that the pond can sustain, we can, and do, stock at much higher levels than would naturally occur. So it follows that we now have a system, which contains much higher levels of waste products than would naturally occur.

Higher stocks of fish and the waste from them means larger quantities of ammonia in the water; but having eliminated the algae and other plants from our ponds, we no longer have access to the very efficient means of removing this ammonia that these plants provide, and so we have to rely upon bacterial activity from beneficial nitrifying bacteria to do this job for us.

A natural balanced system would typically consist of a pond, which would contain plants, fish, invertebrate life and bacteria. A one-acre pond (4,800 sq yards) would typically contain 250 lb fish. A good-sized typical koi pond would have a surface area of 18 sq metres (it is the surface area of the water of a pond which is very important as this equates to the “lungs” of the system, although depth is also important because it confers more stability with respect to temperature change, (more about this later).

It is obvious that koi ponds rarely have as large a surface area, consisting of the pond bottom and sides, as the natural one-acre pond, but we may well have a stock density approaching the same 250 lb of fish stocks. As we are now relying on bacterial activity alone (no plants in koi ponds) to deal with the waste ammonia problem, we now require somewhere for these bacteria to live and perform their good deeds.

The next chapter explains how we may introduce plants into our koi ponds and how the surface areas within a pond can be increased using filtration systems.

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