Dealing with pollution
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.
Solid wastes and koi health
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.
Unseen but there!
Solid waste will ultimately be broken down by decomposer
microorganisms 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.
It doesn't matter whether the solid wastes
decompose in the pond or the filter - the result is the same -
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:
time =filter volume/flow rate
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:
gal / 2000 gal per hour = 0.1 hour = 6 minutes.
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.
To maintain good
water quality It is essential that solids are removed from the
pond and filter before they have time to pollute the
A better way of collecting solids
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
Any trapped solids
must be removed from the system on a regular basis, otherwise
they will simply decompose and pollute the pond. They will also
encourage high levels of opportunistic bacteria.
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.