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 Rocks in the Pond

By Joseph F. Cuny
Reprinted with permission
August 2004 KOI USA
 About the Author: Joseph J. Cuny - 12 yrs Civil Engineering, State of California, B.S. in Physics, University of Nevada, 4 years of Applied Physics in Aerospace, 29 yrs Sanitary Engineering, City of Los Angeles.

 Thank you to KOI USA  and Joe Cuny
for allowing us to use this very timely article

    Some time ago I discussed a practice that seemed to be developing on the East coast. I understand the same practice is now showing up on the West coast. This has to do with the placing of rocks, stones, and/or gravel on the bottom of a pond: it is called aesthetics because it simulates the appearance of natural streams.

    Don't get me wrong, I am not against aesthetics. Yes, I know that scientists, engineers, and so forth are supposed to be cold, calculating things with no interest in the arts, but this is not necessarily so. Even though I am aimed at the engineering aspects of koi husbandry, after many years of koi shows I would hope that some of the aesthetic aspects have penetrated my skull.

    Also, I believe in the truth, whatever that is, and I do not believe in shoving that truth down people's throats. I like to be reasonably sure that people have the facts (not necessarily my facts) before deciding on something. The following are, to the best of my knowledge, the major facts concerning rocks in the bottom of a pond. I am sure that some vendors have their own set of facts; whichever set you want to believe, is up to you.

    Whether the pond is constructed of concrete, fiberglass, or a liner, rocks on the bottom can be bad for your koi. Note the conditional phrase "can be bad" because that is the type of thing often ignored in engineering statements. The various conditions are ignored because people want a simple answer, not one that requires some effort to understand. Come to think of it, the same thing often happens when some koi get sick or die. Usually the owner wants a simple answer, no complications. Unfortunately, life is not simple!

    Now to the question, what's wrong with aesthetic rocks in the bottom of a koi pond? The first reason is the simplest and not even engineering. When koi scratch or flash as they sometimes do, they rub on the bottom. Any projection on the bottom can cause damage to your koi.

    For the engineering answers, we have to consider some of the differences between our ponds and natural streams. In particular, why do we use biofilters on our ponds? Obviously, we use them to process the waste products of the koi and thereby ensure that the pond water is safe. To put it bluntly, we don't want our koi swimming in their own toilets!

    How does this relate to rocks on the bottom of a pond? The rocks interfere with the flow patterns of the water and provide pockets where solid waste can accumulate. (Incidentally, the same problem occurs with a thick layer of algae.) Each of these pockets then becomes an individual sewage treatment plant. Since these rocks simulate those in the natural streams, we have to ask, why are the rocks in the pond different?

    At this point we could chicken out and simply state the answer but let's take a minute or so and slightly expand our engineering knowledge. When engineers design a model, they run into compatibility problems. Real systems such as buildings, machines, boats, ponds, and so forth, are influenced or controlled by a number of things. Most of those things follow different laws-laws that are incompatible in designing models. When the model is designed, the designer must select the aspects to be considered and essentially ignore all others.

    Similarly, if you designed your backyard to simulate a specific Japanese garden, you would not simply scale it down to fit the yard. A scaled-down garden would result in a pond that is probably too small, along with small koi, trees, and so forth; everything would seem inappropriate for a backyard. Again, you would not scale it down but you would design a garden that would give a feel for the original. In other words, your modeling would emphasize the aesthetics of the garden. Now back to the original question, what's wrong with aesthetic rocks in the pond?

    In natural streams, the water velocity (actually speed) is significantly higher than in a typical koi pond. This means flow strength and patterns around the rocks are totally different in the two cases. Because the flow speed and rock sizes in the pond could not be modeled, the pockets of debris around the pond rocks are of different relative sizes and tend to be more stable than in a stream. Additionally, the adjacent water contains significantly lower dissolved oxygen. Note that this is in the pond, not in an aerating stream feeding the pond. In the pond, these sewage treatment plants almost certainly function anaerobically, producing unwanted gasses, bacteria, and so forth, whereas in a natural stream, the treatment would be more aerobic because of the higher speed and turbulence.

    If the rocks were embedded to reduce the possibility of dead pockets, the above engineering reason against the rocks would be eliminated. Such embedment, however, could cause problems of its own. Let's say that you have a liner pond and apply a thin layer of concrete on top of the liner to embed the rocks. What problem can that cause? Over time, organic material dissolved in the water can find its way between the concrete and the liner. Once again, this is an anaerobic environment so the decomposition of the organic material (it's rotting!) produces gasses. Eventually these gasses lift portions of the thin concrete and fracture it. The fractured edges are very bad on flashing koi! Of course, a thick layer of concrete can eliminate this problem but the cost of such a layer usually eliminates the benefits of a liner pond.

    Another condition is sometimes used to prove that the bottom of a pond should have rocks. This condition is the under-gravel filter that is common in aquariums and is occasionally seen in ponds. Note that in both of these cases, they are called filters because they actually are filters, not simply rocks on the bottom. The re-circulating water is drawn down through the rock filter, carrying the fish debris along with oxygenated water. Even in these systems, it is sometimes necessary to clean the debris from the rocks; otherwise, portions of the filter go anaerobic and begin to produce harmful things. If this were not true, why would aquarium stores sell all kind of devices to clean under-gravel filters?

    Oh yes, before I forget it, another consideration exists for liner ponds. If you have high ground water, it can lift the liner! Typical swimming pools are often required to have a pressure relief valve installed in the bottom. This is because such pools are often empty for various reasons, and water pressure from underneath can lift even a concrete pool. In fact, I have seen a filled swimming pool that was lifted about a foot and had to be demolished! The relief valve should have allowed ground water to flow into the pool and relieve the excessive pressure, but it failed. If a liner pond is installed in a region of fairly high ground water, a lot of heavy rocks on the bottom may hold the liner in place. Note, however, this is not for aesthetics but to provide enough weight to counteract the groundwater pressure.

    Now the choice is up to you. Is the aesthetics of rocks in the pond more important than the possible complications introduced? Finally, one other consideration is something called the stocking level. No, not something related to the stockings you wear on your feet, but the quantity of stock (koi) you run in your pond. If you really want a natural pond or stream, you might notice that no natural system carries so much stock per unit volume.

Note - From Aqua Art: This photo shows algae feeding on detritus that is collecting in dead zones created by loose rock. This organic debris builds up and is decayed by undesireable anaerobic bacterial action, which, among other by-products produces hydrogen sulfide. Anaerobic bacterial growth is the type of bacteria used in septic tanks. Pond systems should always strive for environments that encourage aerobic bacteria colonies such as amended rich top soil in a healthy garden - a sweet earthy smell.


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