Cycling of nutrients in koi and goldfish ponds
A koi pond should be designed and operated to get as close as possible to a natural environment which is a tall order because koi ponds are inevitably overstocked compared to any natural and viable lake situation. In essence a koi pond is far more unstable than a natural waterway and changes are magnified dramatically. There is a tendency for levels of undesirable soluble chemicals and organic solid debris to build up and theses must be handled using properly specified equipment if a koi pond and the fish in it are to remain an asset rather than an eyesore.
Water quality in a natural waterway is controlled mainly by self limiting the amount (mass) of fish it can hold. The equilibrium between food availability and biological capacity to handle the waste materials from fish and other aquatic creatures is generally kept in check naturally unless there is interference from an outside source such as pollution or acid rain. The same bacteria that work inside a biofilter are responsible for the breakdown of ammonia and nitrites and plants in the lake absorb nitrates returning nitrogen to the atmosphere. Bacteria to do this cleansing take up residence on any surface including submerged trees, stones and so forth.
Water quality in a koi pond would quickly get out of control resulting in high stress levels and probably death if it were not for the ability of a biofilter to remove toxic chemicals from the water.
Other compounds than ammonia and nitrite need to be considered when it comes to operating a koi pond. Let's take a look at these other compounds and how they impact a koi pond.
Oxygen.
In a natural pond or lake or river, oxygen is produced by water plants and algae through the natural process of photosynthesis by which under the influence of sunlight carbon dioxide or CO2 is absorbed by the plant, broken down into carbon compounds and oxygen with the latter remaining dissolved in the water or if in excess released to the atmosphere as gaseous oxygen. This process is reversed at night and is called respiration. This source of oxygen in the lake's water is supplemented by oxygen entering the water at the surface interface where air and water come into contact. On calm days this process of transferring oxygen is slow while on windy days the rate improves considerably.
In a koi pond where often there are no plants at all and where algae growth is prevented by UV radiation the oxygen is added by forcibly adding air into the pond water system by use of an air pump or venturi (aeration).
Oxygen is continuously removed from the water since it drives the reaction that oxidises ammonia to nitrate.
Carbon-dioxide.
Carbon-dioxide is soluble in water forming weak acids that lower pH. The carbon dioxide comes from the photosynthesis process described above (respiration). Carbon dioxide levels peak during the night. When there is considerable algae in the koi pond excessive levels of carbon dioxide can be produced.
pH drops during the respiration phase as carbon dioxide is dissolved by the water.
Nitrogen.
Most of the nitrogen in a koi pond originates from the proteins found in koi food which ultimately becomes nitrates that remain dissolved in the water. Most tap water sources these days also contain nitrates and are thus a source of nitrogen in a pond. Some nitrates can be removed using aquatic plants and under anaerobic conditions (created in some biofilters) by bacterial reduction of nitrate to nitrogen which is lost to the atmosphere as gaseous nitrogen.
Phosphorus.
Phosphorus as phosphate is found in low concentrations in koi pond water. It is necessary in small proportions to allow full metabolic functions of the koi to take place since phosphates impact upon bone structure as well as enzyme production. Phosphates in general are introduced to pond water via the food fed to koi. Phosphates may also be present in tap water and are taken up by plant growth to a limited extent.
Published with permission Koi Pond Answers
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