Whilst "filtration" on koi ponds has become a somewhat controversial subject full of emotion as people try to expound on their concepts of what should be used to filter koi pond water, there are basic principles that apply to all koi pond filters.




Whilst “filtration” on koi ponds has become a somewhat controversial subject full of emotion, there are basic principles that apply to all variations hobbyists may encounter.

A wide variety of filters and biofilters are available to the hobbyist. Koi Keepers must look past advertising claims, past personal points of view and past complexity. It is a fact that all filters and bio-converters work. The human element transgresses certain natural laws of limitations and the failure of the system results. The terms “filters” and “bio-converters” will be used to describe the devices that remove solids and chemicals, respectively, from pond water. More complete definitions will be given later.

A bio-converter or biofilter is not working and will not work in only two circumstances. 1) when it is new and 2) when it has been destroyed by chemical or medication treatments to the pond or a complete lack of oxygen. In all other circumstances the bio-converter is working to some degree or the other.

Bio-conversion is a simple, natural process in nature. Nature “works”, it’s as simple as that. The number of living organisms that can be successfully accommodated in a limited volume of water (the pond) is determined by the amount of oxygen removed from the water and the amount of toxic waste products added to the water during the process of metabolism by all living organisms in the pond.

Our ponds are a limited volume of water. In order to successfully sustain, in a healthy condition, a large number of koi, we have to re-cycle the same water. The re-cycled water is re-conditioned by removing the unwanted substances such as carbon dioxide, solids and nitrogen waste products then adding in vital oxygen. This purified and rejuvenated water is returned to the pond as quickly as possible for the benefit of the fish and the bio-converter.

It is remarkable that all bio-converters and filters work. The filter system simply has to remove the fish excretion in the water faster than it is added or it will build up to unacceptable levels. The filter must also remove organics such as algae faster than they are produced in the pond or the pond water will go green. We simply have to add oxygen at a faster rate than it is removed. The purified water must return to the pond as fast as possible.


The simplicity of filtration on a koi pond is remarkable. The pond, the piping, the filtration, the bio-converter and the returning water to the pond can be compared to the human body.

Our lungs introduce air into our systems and cells by making gaseous exchange possible. The oxygen allows chemical processes to occur in the cells. This activates our bodies. 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, namely circulation of fluids within the body. These fluids carry a host of vital substances from oxygen to white and red blood cells to hormones to carbon dioxide etc.

No matter how large and strong the heart is (or a pump on a pond) it is useless unless piping, in the form of veins and arteries, are free of restrictions and open for the circulation of blood. 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 that ensures the success of the organism. A failure of any one of several organs will result in the death of the organism. For example, 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 remarkably short time and the organism will die. A healthy circulation system with strong heart, open veins, open arteries, is the key to health within the body. The same principles apply to
our ponds.

The human body is approximately 70 – 80% water. Our ponds are water. The fish are 80% water. The volume of water of the pond is the living environment of 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, and/or venturis for gaseous exchange to take place exactly like the lungs. Oxygen is dissolved into the water and unwanted gasses pas out of the water – 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 for different problems are needed.

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

A bio-converter is a remarkably simple concept. This involves providing enough space for naturally occurring bacteria to grow on. These beneficial bacteria do a vital job in nature and in the pond of chemically changing toxic fish waste to less toxic substances.

It’s as easy as that.

Fascinatingly, the bioconverter is not only home to nitrifying bacteria but home to countless other varieties, many of which have not been identified yet. This incredible diversity of microscopic bacterial life plays a vital role in the success of your 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. 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.

Starting at the Beginning

Viewing the entire pond system with the piping, the pumps, the various aspects of the filter and the returns it is literally a circle. Where do we begin within the circle? After
all, a circle has no starting point and no end. As water is the medium in which fish live we will begin with the water.

Before the fish can be introduced into a pond, before bacterial growth will occur, before there will be any life in the water it has to be activated. Nature activates water or the atmosphere for that matter, with a gas – oxygen. We therefore, have to introduce oxygen into the system as the starting point. Once oxygen is introduced throughout every part of the system. Just as in the body, the oxygen in the blood supply is circulated to every cell and every organ. Should the oxygen supply be cut off from any part of the body for long enough, that area will die.

In order to introduce oxygen into the bloodstream or into the pond water, the water must flow – it must move. Once the water begins to move and there is a flow through the system, oxygen will be introduced into the water with the natural gas exchange of the atmosphere and the system will become active and live. The fish can then be introduced into the system. Once the fish are introduced they will be fed.

Once they feed, impurities will be passed out of the body back into the water. 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 converting 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.

Ponds generate an enormous amount of pollution or unwanted material. This enters the pond system in various forms. Algae accounts for much of the organic build up in a pond. Various other substances such as dust, debris, etc contribute to the build-up of unwanted substances in the pond.

This organic matter ends up in the filter system and must be removed regularly to ensure a healthy environment. The maintenance of a pond system is absolutely critical to the long-term health of the fish. You would not consider trapping ten dogs in a closed courtyard and never ever cleaning it. The pollution these dogs would create in a short time would result in illness, stress and death. Exactly the same applies to fish in the pond.

Down To Specifics

I would like to examine some of the facts and explode some of the myths that have sprung up around filtration on koi ponds. When we explode some of the myths we are entering controversial territory. So let us clearly define what we are talking about and present the facts. Enjoy this essay.

The solids that enter the pond water, even without any koi in the pond, can be dust, atmospheric pollution, wash-off from surrounding soil and the biggest of them all algae. Yes, algae grows even without koi in water. Look at your swimming pool if you do not add chemicals. Solids are removed by mechanical separation or mechanical filtration.

Definition: Mechanical Filters will be referred to as filters. These are areas where suspended and settled solids are collected for removal from the pond system

Myth: Ammonia is filtered from the pond water.
Facts: Ammonia cannot be filtered; it can only be chemically converted from one substance to less toxic substances by bacterial action.
Once we add koi to the pond and feed them they excrete (pass) ammonia via the gills and some solid faeces via the vent. Ammonia is very toxic (depending on pH) to koi. This is removed from the pond water by bioconversion, not bio-filtration. Let me explain.

Once there is ammonia present in the water and there is also oxygen, a species of bacteria found in nature – the nitrifying bacteria – start to grow immediately. These bacteria are chemolithotrophs. They do not filter the ammonia out of the water but chemically convert ammonia to nitrite, in the presence of oxygen. Then after about 6 weeks another species of nitrifying bacteria begin to grow that convert nitrite to nitrate.

The nitrate can then be removed from the pond system via water changes or small pockets of anaerobic bacteria that convert he nitrate to free nitrogen or nitrous oxide.

Definition: Nitrifying Bacteria
Bacterial species found in nature that oxidize ammonia to nitrite and then nitrate. These are chemolithotroph species of bacteria that live in an oxygen rich environment,
utilize mostly inorganic (without carbon) compounds as their energy source, and require carbon dioxide (C02) for their source of carbon. In no part of this process is the ammonia, nitrite or nitrate filtered it is converted to less toxic substances. Hence the definition bioconversion. Bioconversion takes place mainly in the bioconverter. The bioconverter (biofilter in the old terminology) is the place in the pond/filter system where we provide a media for the nitrifying bacterial to grow on.

The nitrifying bacteria are a species of bacteria that need a surface to adhere to. This surface, called a media, becomes coated with bacterial colonies and a biofilm is formed. Provided the nitrifying bacterial colonies, which grow on all surfaces in the pond including the pipe work, are provided with a source of food ammonia from the fish and a source of oxygen in the water, they will flourish and do their job of keeping our pond environment healthy for the koi. It’s really as simple as that.

Definition : Bioconverter / bioconversion – A bioconverter is a specialized or dedicated area of the pond system designed to provide the proper environment for Targe colonies of beneficial bacterial to grow. The nitrifying group of bacteria, the chemolithotrophs, as well as the organic consuming bacteria, the heterotrophs, grows in the bioconverter.

In some literature these areas are referred to as biological filters or biofilters. As the water in the pond becomes older various pollutants enter the water. Some via the wind and some are washed off from the soil surrounding the pond. However, the single largest source of solid pollution in the pond is algae. A single cell algae that can make our ponds go green overnight can reproduce itself 30 times an hour in the sun. Algae grows all the time very day of the year. There are thousands of species of algae.

Some adhere to the walls and produce hair lie growth. If this growth is short all is well. If you have algae that grows particularly long and breaks off then it can get to the pump and cause blockages. But algae does enter the filtration system all the time and is a major cause of sludge build up in all parts of the filter system. Dead algae decomposes and this decomposition removes some oxygen from the system and adds ammonia.

Definitions : Solids

Suspended solids are the non-dissolved pollutants that remain suspended in the pond water. Examples of suspended solids are fine dust, pollen, dead algae and all fine solids and/or those solids near zero buoyancy.

Settled solids are the non-dissolved pollutants that remain suspended in the pond water. Examples of suspended solids are fine dust, pollen, dead algae and all fine solids and/or those solids near zero buoyancy. Settled solids are the non-dissolve pollutants that settle towards the bottom of whatever reservoir they are in. Examples of settled solids are heavier particles of dust, water logged organic material and leaves.

One of the prime pollutants, in terms of quantity, in pond water is dead algae. This accounts for as much as 60 – 70% of the organic material. However, in some lightly loaded ponds, it may be 90% of the organic material that clogs up the filter system.

Lets have a closer look at bioconversion first.

The nitrifying bacteria that do the conversion of ammonia etc. are found naturally in nature. They need a source of ammonia and a source of oxygen. In other words they adhere to a media as well as all surfaces and their food has to be brought to them.

Myth: The bacteria in the bioconverter will die if the pump is turned off.
Facts: This is false. The bacteria are very resilient. They will not die but become dormant. They are easily reactivated when conditions improve.

Myth : The bacteria will be damaged if tap water is used to flush the filters.
Facts: Not true. Once the bacterial colonies have established themselves they form a biofilm. Biofilms are very tough and protective. Results of work conducted by Wirtanen and Mattila-Sandholm (1992b) and Mustapha and Liewen (1989) suggest that the age of a biofilm affects the resistance of microorganisms to chemicals.

A significant fact discovered by all scientist in the research papers on microorganisms I consulted with, indicate that mature biofilms were 150 – 3,000 times more resistant to free chlorine at pH7 and 2 – 100 times more resistant to monichloramine than were unattached cells and new bacterial colonies.

Another interesting point scientists have shown that much higher concentrations of antibiotics are needed to kill bacteria in biofilms, compared to free-living bacteria. Originally, it was assumed that the biofilm provided a physical barrier against the antibiotic; scientists thought that the antibiotic could not penetrate the biofilm. This may play a role in providing protection. However, there is evidence that the nature of the colonies themselves provide protection. By growing in microcolonies, the outer cells protect the inner cells from the antibiotic that does penetrate the biofilm, leaving the inner cells to grow and multiply.

This is fascinating, so lets have a closer look at bacterial colonies. This is critical for our understanding of what is happening in our ponds so lets have a closer look at BIOFILMS