The following article is provided to represent a side of the long controversial question of whether or not to cover the pond bottom with stone. While I am a believer in creating your water feature the way you want, Mr Cuny presents the biological reasons why stone on the bottom creates problems. Appearance is another factor. Whatever surface lines the bottom of your pond will get a coating of algae and detritus. On a dark liner this coating provides a naturalistic appearance. Light colored rocks may look nice for a short time but quickly look dirty in a pond environment.
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, decades ago.
This article first appeared in the March/April 1999 KOI USA. It is reprinted here with permission