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
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
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
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
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.