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Koi pond filtration and fish health
Although there will be some nitrification taking place everywhere in
a koi pond, the sheer volume and rate of ammonia production would soon
overwhelm the system and kill the fish if there wasn't some form of
supporting water quality management. One of the main responsibilities of
the koi-keeper is to control the concentrations of ammonia, nitrite and
nitrate through good husbandry, filtration and water changes.
Solid wastes
There are two main sources of ammonia - dissolved ammonia excreted by
the fish and ammonia produced by decomposing solids. The filtration
system should be designed to deal with both sources and to minimize
their impact on the fish. If all solid waste could be removed as soon as
it was produced this would reduce the source of a substantial amount of
ammonia and other dissolved compounds. Unfortunately, this is not
possible unless we mount a 24-hour guard with nets and pond-vacs - but
we can design and maintain ponds in order to keep solid wastes to a
minimum.
The important point to note is that solids should be removed from the system - not just hidden
from view - otherwise they decompose and pollute the water, irrespective
of whether they are located in the pond or filter.
Regular removal of solid wastes before they decompose is very helpful
in reducing the levels of dissolved pollutants, improving water quality
and discouraging opportunistic parasites and bacteria. The ease with
which this can be achieved depends on the pond design and type of
filter.
Filter types
There are two main types of external filter: the pump-fed and the
in-ground gravity-fed. The cheaper, simpler method is to pump water up
to a free-standing external filter. Having been pumped up, the water
passes down through the filter medium and returns to the pond through a
pipe or pipe work. For very small ponds or those with few fish, this
type of filter can be adequate but there are disadvantages for larger
ponds and those with many fish.
Firstly, the 'dirty' water is drawn only from a relatively small area
leaving most of the solids behind. A secondary form of cleaning, such as
a pond- vac will be needed if the solids are to be removed from the
efficiently. Secondly, those solids that pass through the pump will be
liquidized by the pump impeller, making efficient removal of solids
almost impossible. Added to this is the inconvenience of having to
constantly clean blocked foam pre-filters.
Gravity-fed filters
The gravity-fed system works by pumping clean water from the end of
the filter back into the pond, thus drawing "dirty" water into
the first stage of the filter. This is normally a multi-chamber unit
with a settlement section for the removal of solid wastes and a
'biological' section for the treatment of dissolved wastes.
It is more expensive and complicated to install but it is much more
effective. Because water enters the filter by gravity rather than
through a pump the solids do not get so broken up and can be removed
more easily -assuming the settlement area is designed properly. If the
water feed to the filter is from a bottom drain and the bottom has
sufficient slope, the water is drawn from a wide area, leaving fewer
solids to settle on the bottom.
It is possible to design a 'self-cleaning' pond whereby virtually all
the solid waste is removed from the bottom to the settlement area where
it can be flushed to waste regularly. Obviously, any 'dead spots', where
solids collect, may need regular cleaning with a siphon or pond- vac
Settlement chamber
There are two schools of thought as to whether the settlement chamber
should be fed from bottom drain(s) or from mid-water. This really comes
down to personal choice and depends how well the filtration system has
been designed. Bottom-drain feeding is preferable because the solids are
thereby continuously removed from the pond. The argument against
bottom-drain feeds is that heavily contaminated water can enter the
biological section of the filter and have an adverse effect on
nitrification. Whether this happens depends on the design and type of
settlement or pre-filtering. A large rectangular or cylindrical
settlement chamber will allow most of the solids to settle out by
gravity. If the water then passes through filter brushes or reticulated
foam it will normally be clean enough to enter the biological section of
the filter. A fairly slow flow rate through the settlement area is
important if the suspended solids are to be removed efficiently.
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