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.

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 bio-mass 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.

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.

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 neutralize 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.

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 substances 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.

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 alga growth, which in turn can affect dissolved oxygen levels and pH stability.

And as we all know, the presence of unsightly amounts of string algae 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 realize 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.

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 feces 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.

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 realize that these processes are taking place and if we want to maintain optimal water quality we need to consider the removal or neutralization of all of these pollutants, not just ammonia.

If we ignore the fate of metabolic ammonia for a moment and concentrate on the organic matter, it will be easier to understand how these pollutants can affect the long-term health of our koi if we have a clear idea of what happens to them in the pond. To do this we have to understand a little about fungi and bacteria, Nature's trashmen. We tend to think of microorganisms such as fungi and bacteria as being disease-causing agents but, in truth, relatively few of them are pathogenic (disease causing). Indeed, without the continued decomposing actions of these micro-organisms, the planet Earth would by now be covered with a layer of sewage several miles deep!

We can divide microorganisms into two basic types. First are those that need a supply of ready-made organic carbon to provide them with energy and the building blocks for other molecules such as amino acids. Organisms that require organic carbon, which includes humans, are called heterotrophs. Some micro-organisms and all plants can extract carbon from inorganic carbon dioxide. These are called autotrophs. This distinction between the modes of nutrition of various micro-organisms is important because an environment that encourages one type of micro-organism -for example, autotrophic nitrifying bacteria - may be unsuitable for heterotrophs such as Aeromonas bacteria and vice versa.

Solid organic matter, such as fish waste, is broken down in a series of steps by heterotrophs into progressively simpler compounds. Most of the initial decomposition is done by fungi, which use enzymes to break down the larger, complex organic molecules into simpler, soluble nutrients that the fungi can reabsorb. This process of using enzymes to break organic matter into smaller molecules is carried out by all micro­organisms, each time producing a different organic compound, until finally the original matter is converted into basic non-organic components such as nitrogen, potassium and phosphorous. This whole process of converting organic materials into non-organic matter is called mineralisation.

The ultimate fate of these inorganic elements is to be converted back into organic matter, usually by plants and other autotrophic organisms, and the whole cycle starts all over again. While the biology of decomposition may be mildly interesting, the important point is that it is carried out in many stages, requiring large numbers of different species of micro-organisms that produce a wide range of different organic compounds in the process.

In an ideal situation, the rate of mineralisation would be matched by the production of organic materials and there would be little in the way of free organic carbon compounds (or other pollutants) in the surrounding water. But, unless our filtration and pond husbandry is designed to remove these organics at the same rate as they are produced, there will be a small but often significant level of free organics. As a direct consequence of this mild form of pollution a rising DOC level - the following problems may be encountered:

Excessive alga growth leading to either green water from free-floating algae or dreaded string algae. These aquatic plants (or weeds, depending on your point of view) will thrive on the non-organic products of mineralisation, such as nitrate and phosphates

High levels of heterotrophic bacteria, which is as good a time as any to point out that many of the common pathogenic bacteria (such as Aeromonas and Pseudomonas) are actually opportunistic heterotrophs. This means they are usually present in the pond feeding as decomposer organisms but can, if their numbers are high enough or conditions are right, turn their attention to the fish. The classic example is when fish are stressed and the normal defense systems are weakened. While it would be impossible and undesirable to eliminate these opportunists entirely, I think that most people would agree there is no point in encouraging them either!

High levels of ectoparasites such as flukes and protozoa. These parasites thrive in waters with a high organic load and, because some of them feed on bacteria, the presence of high levels of bacteria will encourage an increase in parasites.

There is some evidence that water with a high organic load can depress the immune system. However, this may be a result of the increase in parasite and bacteria levels, or it could be that some organic compounds produced during mineralisation are stressful to fish.

Organic matter consumes a lot of oxygen while it is being oxidized or decomposed which could, under certain conditions, be detrimental to the well-being of the fish

High organic loads are also implicated in environmental gill disease, a serious and relatively common koi health problem.

Raised levels of organic compounds can make the water look mucky, often resulting in foam being produced at water returns, e.g. filter outlets and waterfalls.

 

In an active koi pond we have two types of pollution; dissolved and solid. If we could remove the solid wastes from the system before they had chance to dissolve and pollute the water we would have better water quality, less dissolved pollutants and fewer health problems.

If we summarize the situation so far, we can see that if we are to maintain the status quo as far as water quality is concerned, we need to remove the pollutants at approximately the same rate as they are produced. We have also seen that the pollution is basically in three forms: dissolved compounds, such as ammonia, inorganic pollutants such as phosphate and DOC, and solid particulate waste.

Decomposer microorganisms will ultimately break down solid waste into a wide range of dissolved pollutants, adding to those already in the system. It makes no difference where in the system these solids decompose - the end result will always be the same, that is, further pollution. This is an important point as many koi-keepers think that once solid waste is out of sight (in the filter) it is no longer a problem.

With the rapid throughput of most filters, the dissolved pollutants produced as these solids break down are quickly pumped back into the pond. What we really need is two filtration systems - one that enables us to remove the solid wastes from the system before it has time to pollute the water and the other to deal with the dissolved pollutants. After all, if we could remove waste solids from the system, we would prevent most of the sources of pollution.

So perhaps, we should look on our filter as a system of two parts, one part dedicated to removing solid waste matter from the system (not just the pond) and the other removing dissolved pollutants.

Remember, too, that the pond is also part of the filtration system, as a significant amount of mineralisation and nitrification will take place on surfaces within the pond. The pond will also act as a settlement chamber for solid wastes, which will need regular removal to prevent them polluting the water. It is my opinion that the regular removal of accumulating solid wastes presents the koi-keeper with his or her biggest challenge and a great many problems will be avoided if this can be done effectively. For regular disposal of solid waste there are essentially two practical options: settlement and entrapment.

Settlement areas or chambers have to be fed by gravity-flow systems, ideally via a bottom drain. This way, solids are moved gently to a collection area, ready to be flushed out of the system. The traditional method required a large settlement chamber and these can be very effective provided that the chamber is large enough and the flow rate is low enough to give lighter solids time to settle. The retention time for water in the chamber is important. The retention time is simply the filter volume divided by the flow rate, thus:

For instance, if a filter has a flow rate of 2,000 gallons per hour and the settlement chamber holds 200 gallons, then the retention time will be:

It has to be said that, in the above example, the short retention time of 6 minutes is unlikely to be satisfactory, whereas a chamber volume or capacity of 300 gallons would give a retention time of 9 minutes, allowing much better settlement.

Having collected solid wastes, it is important that they are flushed out of the system regularly, before they have time to decompose. During summer this could be as often as twice a day, and obviously less frequently in winter. This means that the settlement chamber will need to have a drain to facilitate easy flushing to waste.

An increasingly popular settlement option nowadays is the cylindrical chamber with conical base that promotes a slow swirl of water. The cylindro-conical shape encourages settlement of wastes into the bottom of the cone, where they collect together, making removal to waste simple and efficient. Again, retention time is important and a slow throughput will be more successful than a chamber that resembles a vigorous whirlpool. One should be guided by the manufacturer as to the right size for your systems but, if in doubt, err on the large side.

When set up correctly these chambers work well and are probably superior in practice to rectangular settlement chambers of similar dimensions. Both types need additional cleaning if solids are not readily flushed to waste since solids may cling to the sides and there may be areas of poor water flow or 'dead spots'.

Last but not least is the pond itself. Even the best-designed pond seems to have dead spots, where mulm and fish waste collect. Any waste that isn't drawn through the bottom drain will need to be removed from the pond before it pollutes the water and there are several options, depending on the pond design. It could be gently pushed towards the drain with a soft broom; the waste could be carefully removed with a fine net; or the pond could be vacuumed. In most cases it is probably a question of combining all three actions, with most ponds benefiting from a regular vacuum during summer.

The most common entrapment systems consist of filter brushes or sheets of foam. We should consider what type of bacteria likely to be attached to the brushes or foam, which are heavily loaded with trapped solids? Common sense tells us that it is going to be heterotrophic bacteria, and do we really want to encourage high levels of these bacteria in any part of the system? You will recall that many heterotrophs are also opportunistic pathogens and are quite happy to lunch on our koi - given half a chance! Obviously the answer is No. So the important thing with entrapment is that the entrapment media are kept clean, otherwise they themselves become a source of pond pollution.

This reminds me of a case last year, when several fish in a quarantine tank became ill. The tank was spotless yet the fish had parasites and were suffering from the onset of bacterial problems. Further investigation showed that while the tank was exceptionally clean, the filter wasn't. When we took the media out for cleaning, the smell was overpowering. The media were covered in a yellow slime, which, of course, was all solid fish waste slowly rotting down. In this case, the filters were slowly poisoning the fish. Following a good clean out of the filters, the fish were soon back to normal

By using a combination of settlement and entrapment it is possible to remove a lot of solids from your pond before they rot down provided, of course, that these areas are cleaned regularly. If we are successful in removing solids from the pond before they pollute the water we are part way to achieving the ideal of unpolluted water.

	encourage a vigorous growth of nitrifying bacteria
	reduce the load on the biological section
	the three major factors affecting our choice of filter media are:
	specific surface area
	void space
	cleanability

	'gin-clear' water,
	breakdown & removal of DOC,
	conditions which discourage filamentous algae (string algae),
	generally optimal water conditions for fish.