Picture courtesy of National Geographic

Pond filtration is remarkably simple. It is not difficult to imagine the pond, the piping, the filters, the biofilter and returning the water to the pond being compared to the human body.

Our lungs introduce air (oxygen) to our system and cells making gaseous exchange possible. The oxygen allows chemical processes to occur in the cells. The liver is a filter removing toxins and impurities from the blood. The kidneys, colon and skin excrete harmful by-products to waste.

There is one golden thread that enables all the functions to work – the circulation of fluids / blood within the body. These fluids carry a host of vital substances from oxygen to white and red blood cells to hormones to cell waste products such as carbon dioxide to nutrition etc.

No matter how large and strong the heart is (or a pump on a pond) it is useless unless the veins and arteries (pipes) are free of restrictions and open for the circulation of blood (water). The blood that carries nutrition and oxygen into the cells also carries waste products away. Within the blood, a host of cells circulate to assist with the immune system, the nutrition and the oxygenation of the body cells.

It is an effective circulation system that ensures the health of the organism. It is an effective circulation system that will ensure the health of your pond and ultimately the health of your Koi.

A failure of any one of several organs could result in the death of the whole organism. Liver failure will result in the body being unable to filter and remove impurities from the bloodstream. The body will then die. With kidney or bowel failure the same thing - impurities in the body will reach toxic levels within a remarkably short time and the organism will die. A healthy circulation system with strong heart, open veins and open arteries, is the key to a healthy body.

The volume of water in the pond is the total living environment for our fish. The pond water is piped (analogous to the blood vessels)to a pump (the heart) and through various filters (equated to the liver, kidneys etc. of the body) for purification and rejuvenation. The pond water is brought into contact with the atmosphere via waterfalls, streams, air stones with air blowers and/or venturies where oxygen is dissolved into the water and unwanted gasses are released or de-gassed into the atmosphere just like the lungs.

Failure of any one of these systems will result in the death of the pond. When the pond dies, the fish will die.

The purpose of any filtration on a Koi pond is to remove unwanted substances from the water – continuously and quickly. Just as removing the sugar and the tea leaves from a cup of tea would need two different approaches, so too in a Koi pond different approaches are needed for different problems.

Impurities that are dissolved in the water need chemical removal by bacterial action (bioconversion) or with charcoal or zeolite. Impurities in the form of solids need mechanical removal (filtration).

A biofilter is a remarkably simple concept. This involves providing enough area where there is enough space (surface area) for naturally occurring bacteria to grow on and in sufficient numbers. These beneficial bacteria – the nitrifying bacteria - do a vital job in the pond by chemically changing toxic fish waste, in the presence of oxygen, to less toxic substances. It’s as easy as that.

Fascinatingly the biofilter is not only home to nitrifying bacteria but home to countless other varieties of bacteria, fungi and micro organisms. This incredible diversity of microscopic life works in synergy and plays a vital role in the success of your pond. For example, the heterotrophic bacteria that live off the organics in the pond grow much more vigorously than the nitrifying bacteria and take up more space. Make provision for this in the form of good mechanical filters before the biofilters as well as a good maintenance program (in both the mechanical filters and the biofilters).

Losses caused by restrictions in unhealthy veins and arteries in the body compromise the circulation and the health of the person. Exactly the same occurs in a pond. Piping that is clogged or piping that is too small or even piping with numerous bends will restrict and reduce the flow rate, no matter how large the pump. These losses can be substantial to the point of compromising the design of the system.

Starting at the Beginning

View the entire pond system with the piping, the pumps, the various aspects of the filtration system and the returns as a circle. Where do we begin within the circle? After all, a circle has no starting point and no end. The entire pond/water/filtration system must be viewed as one environment.

Before the fish can be introduced into a pond, before bacterial growth will occur, before life in the water will flourish, there has to be oxygen present. As Koi keepers our responsibility is to introduce oxygen into the system as the starting point. Once oxygen is in the water it must be circulated throughout every part of the system. Just as in the body, the oxygen in the blood supply is circulated to every cell. Should the oxygen supply be cut off from any part of the complete pond system for long enough, that area will die.

High oxygen levels in Koi ponds are a key element for good Koi keeping.

In order to introduce oxygen into the pond water, the water must flow - it must move. Once the water is flowing through the system and around the pond, oxygen will be introduced with the natural exchange of gasses when the water comes into contact with the atmosphere and the system will become active and alive.

The only way oxygen can be dissolved into the water is through contact with the atmosphere.

Once the system is active and free of chlorine and chloramines, fish can be introduced. As soon as the fish are introduced they will begin to excrete impurities into the water as a natural part of metabolism. The combination of oxygen and excretion from the fish results in the growth of naturally occurring nitrifying bacteria. These bacteria will grow on all surfaces within the pond. There they begin their job of using the ammonia as a food source and convert the fish impurities to less toxic substances. Bacteria take time to reach large enough numbers to convert all the ammonia to nitrite and, at a later stage, to nitrate.

It is as easy as that.

It is also a fact that ponds generate enormous amounts of organic matter in the form of algae. Various other pollutants such as leaves, dust and debris,contribute to the build-up of unwanted substances in the pond.

The organic matter ends up in the filter system and must be removed regularly to ensure a healthy environment. Organics not removed from the pond biodegrade. This process uses oxygen therefore, the available oxygen levels are reduced and there is competition with our Koi for the critical oxygen in the water. The by-product of rotting organic material is ammonia so the ammonia levels rise. You can have a pond with no Koi, low levels of dissolved oxygen and a high ammonia level!

Any area with large amounts of organic material build-up will have layers forming. The deeper layers will be cut off from the passing oxygen in the water and become anaerobic. A by-product of anaerobic bacteria is hydrogen sulphide which is highly toxic to Koi even in minute quantities.

The maintenance of a pond system is absolutely critical and the key to the long-term health of the fish.

Arm yourself with knowledge and always keep to the KISS principle – “Keep It Simple Stupid”