AGAINST ROCKS IN THE POND
The Case for Smooth Bottom Pond Design:
Todo --- North Florida Koi Club,
Past President & News Letter Editor

Recently there has been an ongoing debate on the news group rec.ponds
discussing several ponders preference for ponds with a layer of gravel
across their pond's bottoms. Among their reasons for this are 1) The
light bottom makes the fish easier to see, 2) the gravel is more
attractive than a smooth black liner, 3) what possible difficulties that
may occur due to trapped organic matter can be solved by biological
and/or chemical means.
There are however, real reasons to reconsider the decision to place
gravel in a pond.

The bottom may be lighter in color, but often times this light rock gets
covered with dark green algae, which defeats the intention. The gravel
can get covered with mulm, the dark organic material that is a byproduct
of the natural biological functions of a pond. The mulm can totally or
partially obscure the bottom. Hence, it is an open question how long the
bottom will stay pristine.

As for the aesthetics of the gravel. There is not discussion on taste so
this becomes no real argument for or against this question.

The third point is then the most critical and most contentious. With a
gravel bottom the mulm, fish waste and decaying plant matter from the
pond does not flow quickly to the drain or sump, where it can be removed
from the pond. The gunk gets trapped in the spaces between the stones
and there it sits.

In a properly designed pond the solids produced in it are carried by the
flow of the water down to where the pump can suck the material out and
send it to the filter system where it belongs. (Please note that I do
not imply that a pond with gravel is by definition poorly designed, for
there are many smooth bottom ponds whose specifications do not add to
its efficiency.

However, I do say that no matter what the design, gravel degrades the
efficiency of a pond).

What then, is the detrimental effect of having gravel. The first thing
to remember is that a pond is not an aquarium outside. Gravel, used in
an aquarium has a purpose, it acts as part of the filter system. Water
is drawn through the gravel, mechanically removing debris and providing
a surface for aerobic bacteria which neutralize ammonia and other
byproducts of the aquarium system. The water is then pumped back into
the tank.

In a pond there is not under-gravel filter. Water is not drawn through
the stones, it passes over the top and the mulm that accumulates under
and between the pebbles builds up in an oxygen deficient environment. As
a rule, aerobic bacteria are beneficial and anaerobic bacteria are not.

Anaerobic bacteria survive without oxygen and their byproducts are often
harmful, such as sulfur dioxide, the chemical that give a rotten egg
smell. In addition to these bacteria, other harmful organisms fester in
the goo at the bottom of gravel ponds.

To counter the foulness of the gravel bottom, practitioners say that
chemicals can be used to reduce the mulm to an inert material. There are
other products, such as PondZime which reduces the mulm with enzymes.
The efficiency of these remedies is questionable, not to mention that
even if the crud is neutralized it still continues to accumulate. Hence,
more chemicals must be used to deal with the problem. It seems that a
much better, less costly, and far preferable solution is to have the
pump remove the gunk.

Here, a counter argument says that the mulm is only deposited in the
filter and so it doesn't matter where it sits, in the filter or on the
bottom, it is still in the pond system. This argument fails when the
mechanisms of filtration are known. For in a filter system, water is
passed though the filter, supporting aerobic bacteria just as it is in
the under-gravel filter in an aquarium.

So the issue comes full circle just as water is pumped from a pond, just
to be filtered and returned. The idea of having gravel comes from
aquariums, but as ponds are different than aquariums the function of the
gravel is shifted to the filter system in ponds. Ironically and in
opposition to the logic of some ponders, the gravel becomes detrimental
to the pond.