At the heart of the koi pond
At the heart of a koi pond is the filtration system. To encourage a
vigorous growth of nitrifying bacteria in the filter we need to keep the
filter media fairly clean, which is often easier said than done!
Efficient biological filtration also depends on the media of choice
having an adequate specific surface area (SSA), adequate voiding and
water retention time.
For good filtration and good water quality very little solid waste
should enter the 'biological' section of the filter. Although, for
simplification, I make a clear distinction between the settlement /
entrapment areas of the filtration system and the biological section, it
is worth remembering that the two are linked, so some nitrification
occurs in the settlement area, and some settlement and mineralisation
occurs in the biological section.
However, the aim is to ensure that conditions in the biological
section are such that vigorous growth of important nitrifying bacteria
is encouraged, while conditions that would promote growth of
heterotrophic bacteria are avoided. Most likely, heterotrophic bacteria
will predominate at the start of the biological section, where the
smaller particles of remaining solid organic matter become trapped. So,
in general, when we refer to the biological section we mean the area
where dissolved pollutants and particulate matter are converted by
microbes into less harmful substances.
To re-state the point made on other pages; the more effective the
settlement area of the filter at removing solid waste, the lighter the
load on the following biological section - provided, of course, that
trapped waste is removed before it decomposes.
More than nitrification
I made the point that a filter does more than simply convert ammonia
to nitrate. Although ammonia is often the most toxic pollutant in a
pond, we should not forget that normal biological, metabolic and
chemical activity produces a wide range of pollutants. Even with the
most effective settlement and entrapment system many organic wastes,
particularly fish faeces, will start to decompose, producing various
dissolved organic carbon compounds (DOC). We expect the filtration
system to deal with these pollutants also, not just with metabolic
The diverse range of biochemical processes occurring in the filter
are due to many species of bacteria, fungi, protozoa and various worms
and possibly snails present- not just nitrifying bacteria. However, our
pond husbandry routine should be designed to maintain conditions which
encourage nitrifying bacteria, and this is achieved by:
regular maintenance to keep the
biological area clean and free of mulm,
reducing the level of dissolved
organic compounds by effective settlement/entrapment, together
with regular cleaning of the settlement area.
If we can remove solids from the system before
they decompose and at the same time keep the biological section
of the filter fairly clean we will;
Filtration how & why
Understanding the fundamentals of biological filtration is helpful in
diagnosing how common fish health problems occur. However, before we
look closer at what goes on in the biological section of a filter, it is
worth considering other important aspects such as filter size,
efficiency and overall design. There are many rough guides to determine
required filter size and flow-rate.
One such suggests that the filter surface area should be
approximately one tenth that of the pond; another, that there should be
a pond turnover rate of once every 2 to 3 hours. While a rough guide is
helpful, the huge variety of different filter media and designs
necessitates a more specific approach. When it comes to water quality
there are three main reasons for why things go wrong:
indiscriminate use of chemical
treatments, which can damage filter function
poor maintenance of pond or
poor filter design
It is the latter problem that we need to address first, by
considering exactly what we expect from a filter and what are the basic
parameter values needed for its optimal performance.
Probably one of the most discussed subjects in the hobby of
koi-keeping is the merits of various filter media - and there are
sometimes quite incredible claims made for the various types. But the
first thing to be clear about is that bacteria will thrive on almost any
surface and the particular choice of medium has very little
influence on their growth.
How much surface area?
Nearly all types of filtration system rely
on attached-growth processes in which a bacterial slime layer or biofilm
-comprising bacteria, algae and often larger invertebrates - forms on
the media. Microorganisms present in the biofilm 'feed' from water that
flows past. So, as a first approximation, the amount of biological
activity will be determined by the amount of available surface area for
bacterial colonisation. However, in practice this available specific surface area (SSA), as it's
called, is rarely a limiting factor since most filtration systems are
Obviously, if you had just a square piece
of material measuring say I m x 1 m this would give a total area of two
square metres (with both sides being available for bacterial
colonisation and assuming almost zero thickness). Even this small area
could support millions of microorganisms, attached in a slimy biofilm.
But typical filter media have a far greater SSA. For instance, gravel
has an available surface area of about 100 to 200 square metres per
cubic metre (100-200 m2/m3). And other, more
specialist media can have significantly more surface area; for example:
So we can see that even a small amount of filter medium provides a
potentially vast SSA for bacterial colonisation. Each square metre of
biologically active surface can metabolise nearly one gram of ammonia
per day, dependent on temperature, and given that most ponds will
usually be producing fewer than 10g of ammonia per day, the amount of
SSA required is really small - and not a lot of people know that,
as Michael Caine might say. If we based filter sizing on the basis of
SSA alone, filters could be incredibly small -perhaps the size of a
shoebox! However, there are other factors to consider ....
Is void important?
The void size or empty space within a filter medium is important in
determining the right filter size and efficiency. Void size is a measure
of how much of the medium consists of empty space. If we consider sand,
for instance, each particle has a large surface area in relation to its
volume and the total SSA per cubic metre of sand works out at thousands
of square metres. Despite this enormous SSA, sand would make a poor
filter medium because the small particle size would soon lead to
blockages and subsequent 'tracking' as water found the 'easy routes'
round the medium. And, of course, because of the dense packing,
any flow through the sand would be very slow. So, despite its massive
surface area, once compacted and blocked the amount of surface area
exposed and the volume of water that could be treated per hour, would
actually be quite small.
There is another important disadvantage of a medium like sand - retention time, or the amount of time the
water spends in contact with the biofilm. It is obvious that if we wish
to avoid blockages and tracking, some void space in the filter medium or
media is desirable. If we consider a medium such as gravel, although its
larger size yields less SSA it is less prone to tracking and blocking.
And specialist media such as filter matting, plastic or sintered glass,
have both a large SSA and a generous void space. In fact, many of them
are more than 90% void or empty space! This makes tracking and blockage
What about cleaning?
Another important consideration - which becomes more important the
longer you keep koi!- is ease of cleaning.
In the early days of the hobby, part of the novelty lies in spending
weekends cleaning and vacuuming. But after a while, strangely, it seems
that there are more pleasurable ways to spend a sunny Sunday. And with
gravel and other granular media, it really isn't much fun trying to
clean several tons of the stuff! Compared to gravel, cleaning
light-weight media is a delight. Obviously, regular maintenance is
somewhat easier if each filter chamber has its own bottom drain but,
even so, ease of maintenance has to be a major consideration in the
choice of filter medium.
the three major factors affecting our choice of
filter media are
specific surface area