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 It has been brought to my attention
that some pond builders (both Koi and water garden) apparently
are not familiar with basic pond design.
In particular, I am referring to the use of gravel
and rock inside the pond. As anyone who has been a pond
keeper for a few years knows, that is a
real no no. Possibly this practice is a direct carryover
from the aquarium 'under gravel' filters, but it is a wrong application
of the concept.
The problem is that the gravel and rock form a dead zone, more
appropriately called a stagnation zone. Whether the pond has
fish in it or only plants, there are all kinds of organic material
produced in the pond. This organic material gets trapped in the
stagnation zone and guess what happens? It stagnates! Since the
water does not circulate very well in this zone, the water is
deficient in dissolved oxygen, and the organics are processed
anaerobically. In other words, the organic
material rots or putrefies.
The end result of such putrefaction
is the production of noxious gases and
disease organisms. I do not know what effect these would
have on plants but they are deadly on fish
and are very unaesthetic. In time such a pond would smell like
a cesspool! This is exactly what happens in an aquarium if the
under gravel filter is not cleaned regularly. Even with oxygenated
water flowing through such a filter, there are dead spots where
the water does not flow and these have to be cleaned, usually
with some type of vacuum or siphon system.
If it is necessary to put rocks
on the bottom of a pond, possibly to simulate a natural stream,
they should be well separated and bedded in mortar to allow flow
around them and to eliminate pockets where debris could accumulate.
The amount of work necessary to properly imbed gravel probably
eliminates the use of gravel. If it is thought that the gravel
will function as a filter, it should be placed such that it can
be fairly easily cleaned. Despite claims
made by various people, all filters must be cleaned. The
only possible exception is the trickling filter type that is
self cleaning, but in the process dumps the waste into the following
stage where it must be captured and disposed of.
I have seen gravel filters built
into a sump in the bottom of a pond. Such a filter is almost
impossible to clean, and the resulting pond does not provide
a healthy environment for the fish. With such a filter in a water
garden, I would expect the same type of problems. I
realize that many people believe that the root system of the
plants provides space for the organisms that degrade the organic
material, but this is not a suitable filter except for those
organics that are soluble. Even these, when aerobically
digested , result in material that falls to the bottom where
it joins the leaves, stems, decaying roots, and so forth and
then putrefies.
Joe Cuny is a founding editor and contributes to
KOI USA since its beginning, over 20 years ago.
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