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Koi pond filtration and fish health?

A biological filter extends the capabilities of what is an unnatural and otherwise inadequate system by continuously treating the water and removing some toxic compounds - ammonia and nitrite being the most common. However, as we all know, there are many different types of filter, some more efficient and effective than others. So which are best? This is a deceptively simple question without a simple answer. What we can do, though, is to look a bit more closely at how a filter works and what sort of load is placed on it and thereby arrive at some basic guidelines.

Do we really need a filter?

If we consider a range of ponds, from the standard goldfish pond with just a few relatively small fish and lots of plants, up to the other extreme - an overstocked koi pond containing several dozen large fish - the weight or biomass of fish per unit volume can vary dramatically. The goldfish pond may have only a few grams of fish while the koi pond may contain several kilograms worth.

It is the total weight of fish or the biomass which determines whether filtration is needed and what type of filtration system is needed.

The filtration or purifying requirements for these two very different extremes will be completely different. Indeed, the goldfish pond may manage quite adequately without any additional filtration to that naturally available within the pond itself. The plants in the pond would remove the small amount of nitrogenous waste produced by the fish and there would be adequate surface area in the pond for the relatively small numbers of nitrifying bacteria. Many owners of small ponds like to have a waterfall and it is then no problem to run water through a small filter, mainly in an attempt to produce clearer water. In a low stocking situation a filter may be an optional extra and the amount of beneficial biological filtration produced is liable to be small.

Koi are efficient sewage making machines

In contrast, in a typical koi pond the amount of waste produced by more and larger fish means that the water is continuously being polluted to higher levels, which in turn will have a serious affect on fish health. Indeed, with no filtration the fish would soon generate sufficiently high levels of pollution to kill themselves. In this situation filtration is not an optional extra, but a vital necessity. If the filter also helps produce clearer water that is an added bonus.

I view koi as extremely efficient sewage-making machines. We throw in high quality, expensive food and the koi convert it to high quality water-polluting sewage. If we are to maintain good water quality, we need to remove or neutralise the sewage as fast as the fish produce it, otherwise there will be a steady increase in unwanted pollutants. This is an important point that I will return to later.

What is pollution?

At this stage we should clarify what we mean by pollutants and at the same time widen our expectations of an adequate filter system. A simplistic view of filtration is the conversion of toxic ammonia into less harmful compounds. While this may reduce or remove any potentially harmful toxins, it doesn't necessarily result in unpolluted water. 

Ultimately, what we are trying to achieve and maintain are optimum water conditions that are as near as possible to those that the koi would find in their natural surroundings. Any chemical or substance present at higher than normal levels, even if it is not directly toxic, should be considered a potential pollutant. So our filtration system, together with our routine pond maintenance schedule, should be designed to remove not only toxins such as ammonia and nitrite but also the products that result and often accumulate when these initial metabolites are degraded. If we are going to extend our view of filtration to include the control of non-toxic waste products, it is obviously going to mean a completely different approach to our previously simplistic view of water quality, filters and routine pond husbandry.

Ammonia and nitrite are not the only pollutants produced by fish. Any judgment of water quality should take account of total water pollution - not just ammonia and nitrite

Background, non-toxic pollution affects koi health and encourages disease

Why should we make life difficult for ourselves in bothering about these non-toxic pollutants and what are they anyway? A short answer as to why we should be concerned about the level of this type of 'background pollution' is that it can indirectly encourage increased levels of both bacteria and parasites and it has also been implicated in lowering resistance to infection. It also encourages algal growth, which in turn can affect dissolved oxygen levels and pH stability.

And as we all know, the presence of unsightly amounts of blanketweed can encourage the koi-keeper to start loading the pond with yet more chemicals in an attempt to eradicate the problem. So although many of these pollutants are themselves not directly toxic, they can be indirectly involved in many of the more common health problems.

If we again make a comparison between the lightly stocked goldfish pond and the often overstocked koi pond, and ask which system is more prone to health problems, the answer must surely be the koi pond. The main difference between the two, apart from stocking levels, is the background level of non-toxic pollutants. A better understanding of these pollutants requires a change in the often over-simplified view of water quality. The conventional and popular view is that the fish produce metabolic ammonia and all of the fish waste and mulm also breaks down, in a single step, to ammonia. In the filter, these copious amounts of ammonia are converted to harmless nitrate - end of story. But that is only the beginning.

Firstly, we have to realise that fish food is concentrated, containing high levels of protein and other nutrients. This means that a relatively small amount will have a large polluting effect. This could be demonstrated quite convincingly by simply placing a couple of pellets of fish food into a small container of pond water for a few days and then testing the water sample for a range of parameters. One of the major changes is a two-to-threefold increase in the level of dissolved organic carbon (DOC), which indicates an increase in pollution. (It is referred to as carbon because the basic structure of all organic molecules such as proteins, fats and carbohydrates, is based on carbon atoms). The other noticeable effect of this little experiment is a dramatic increase in phosphate, an ideal plant food.

Food in, sewage out

The effectiveness of a fish's digestive system is directly related to the availability of food. Therefore, when food is scarce or of low nutritional value, as it would be in the wild, the digestive tract extracts the maximum amount of nutrition from the food. The resulting excreta will therefore be fairly well degraded, bearing in mind that, in the wild, the food was liable to have been of relatively low nutritional value in the first place. However, when food is plentiful or rich, the digestive tract gets lazy and tends to absorb only enough for it's immediate requirements, so the faeces are likely to contain quite high amounts of undigested nutrients, or pollutants as they have now become. So you can perhaps accept my analogy between koi and sewage machines - food in one end sewage out the other.

It is important to appreciate that fish waste is in many respects similar to solid food, inasmuch as it still contains proteins, nucleic acids, fats and carbohy­drates. Because it still has a high organic content, this type of semi-solid organic matter is called particulate organic carbon (POC). When this waste is further degraded by decomposer organisms part of the protein content will be ultimately converted to ammonia and other nutrients will be progressively broken down into sugars, organic acids and a whole range of simpler organic compounds.

Ammonia - a toxic by product.

Perhaps at this stage I should clarify what is meant by metabolic ammonia. This is ammonia produced in the fish's body as a result of breaking down amino acids for use as an energy source. This involves a process called deamination, taking place in the liver, during which the amine chemical is removed from amino acids. All animals produce metabolic ammonia but, as it is such a toxic compound, virtually all other animals, including humans, immediately convert it into a less toxic substance before it is excreted. Humans convert ammonia into urea, which is passed out of the body as urine.

Fish 'don't bother' to convert ammonia, they simply excrete it continuously from their gills into the surrounding water. In a natural environment the immediate dilution by thousands of gallons of water would render it harmless. However, no-one told Mother Nature about koi-keepers and their ponds so it's not quite the case in koi ponds, where ammonia can build up to a dangerous level because of the large number of fish in a small volume of water.

So we can see that pollution in a pond has at least three sources: metabolic ammonia (the quantity being determined by the number of fish and how active they are), inorganic compounds (such as phosphate) and various dissolved and particulate organic carbon compounds. Understanding the chemistry is not important here, it is enough simply to realise that these processes are taking place and if we want to maintain optimal water quality we need to consider the removal or neutralisation of all of these pollutants, not just ammonia.

A koi pond has at least three major sources of health-affecting pollution.

  • Metabolic ammonia
  • Inorganic compounds produced during decomposition of fish waste etc.
  • Dissolved and particulate organic compounds from decomposing organic matter


Fish disease problems? Need some help? The help pages take you through the basics of the 'fish health work-up' diagnosis method and basic fish disease treatment methods.

90% of all health problems and fish deaths are caused by poor fish keeping! The key pages are essential reading for all beginners and intermediates who are serious about disease prevention.

Wife Beater

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Expanded Content by Dr. Erik Johnson, DrJohnson.com and Used with Permission; Frank Prince-Iles ©2009 All Rights Reserved