Beware the Stagnation Zone in Your Pond
Gravel and Rock Form Dead Zone
by Joseph F. Cuny
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.
This article first appeared in the March/April 1999 KOI USA.
It is reprinted here with permission.