Koi Digestion Excretion

The ‘living Koi pond’, whether a clear and circulating or natural and clay-based, consists of a living highway of biological processes that interact with each other to form an aquatic environment. In a natural water body, food and energy will cycle through a chain of closely associated organisms that share in the strategy of ‘eat and be eaten’. Whereas, in an artificial koi pond, we prevent the food chain from cycling, chopping it at the start (by offering man-made koi food) and curtailing it at the end (by the cleaning and purging of filter systems).

Yet in each eco-system, whether natural or artificial, koi are found at the top of the food chain, (a position usually reserved for a carnivore rather than an omnivore) and in doing so, set the agenda for those organisms that scavenge off the by-products produced and excreted by koi.

Two fundamental processes performed by koi in effect prime the rest of the ecosystem, where other organisms lag behind and react to the actions of koi. These processes are digestion and excretion.


Digestion is a biological process performed by all animals that consume solid food. The objective of digestion is to breakdown insoluble nutrients (such as proteins, carbohydrates, and oils) into their soluble building blocks that can then be dissolved in the blood and transported with ease around the whole body. Digestion is usually performed by larger organisms with smaller organisms such as protozoa and bacteria largely absorbing pre-digested soluble nutrients (such as amino acids, sugars, and fatty acids).

In koi, digestion can be divided into mechanical (physical) and chemical processes, each of which acts to break down complex, soluble food items into their soluble components. Just as is the case with filtration, the first part of digestion is mechanical involving the physical grinding action of the koi’s pharyngeal teeth. Located in the throat area, the pharyngeal teeth act like a mill, ‘chomping’ upwards, grinding food against a hard, calcified pad located at the top side of the throat area.

When feeding on a natural diet of worms, diverse vegetable matter and tough decaying organic matter, carp teeth have to work hard to break up the structure of these tough food items. Koi are comparatively spoilt when fed on finely milled, extruded pellets, that although hard, break up relatively easily into the finer flour-like particles that were milled during the food’s manufacture. Furthermore, the feeding habits of artificially reared koi put pressure onto fish to ingest pellets whole; it’s either that or go without food.

Koi that graze naturally in a clay pond will forage at their leisure between supplementary feeds, allowing them to take longer, if required, to process and mechanically break up the more resilient food materials.

Once ingested and milled by the teeth, the finely processed food enters the intestine where it undergoes chemical digestion. Unlike the vast majority of other vertebrates, koi do not possess a stomach (which in predators is used to receive and ‘digest’ a kill) and so lack the benefits of acid digestion. It is for this reason that koi benefit from a steady flow of small portions, something that their intestine can accommodate far better that a few feeds a day on which koi are forced to gorge themselves.

Chemical digestion in koi relies on a host of enzymes (bio-chemicals) to break down the food groups into soluble components. Koi secrete different enzymes along the intestine to digest different nutrients. A different enzyme (or groups of enzymes) work specifically on each nutrient eg. proteases break down proteins, and lipases breakdown lipids etc). Once soluble nutrients become available, the objective of digestion has been completed as koi are now able to absorb them through their intestine wall and into the blood stream.

Yet as no living organism is 100% efficient, the digestion of food is never complete, neither is the absorption of the products of digestion, leading to differing degrees of excretion.


Excretion is not an end in itself, but represents a feeding opportunity for other organisms. The excretory products of koi will include the non-desirable by-products of their metabolism (ammonia) as well as solid and soluble organic waste that has passed right through their gut. Although we (and koi) regard waste products as useless, there is a whole host of organisms ready and waiting (and dependent) on processed, digested (and therefore readily absorbable) organic matter that koi excrete.

The quality and quantity of food we offer and koi that are stocked will have a direct bearing on the numbers of lower organisms that can be sustained by the excreted and pre-digested products of koi digestion.

If we give our koi too much food, the efficiency at which they can digest it decreases (leading to greater excretion and an abundance of scavenging organisms). Similarly if food offered to koi is too ‘rich’, (i.e. high protein) relative to the temperature, then due to a koi’s reduced ability to digest and absorb nutrients, more food will be returned to the pond unabsorbed by a koi’s intestine. This has significant consequences for us as koi keepers as elevated levels of excretory products cause water to deteriorate both directly through the build up of organic material and indirectly through the increased demand for oxygen that these processing organisms can put on a pond’s oxygen budget.

In a highly filtered, clear koi pond, the risk of high levels of koi waste is reduced by regular removal of even the finest of solid organic material. If allowed to accumulate in a filter, micro organisms would proliferate and threaten the stability and ‘sweetness’ of the pond’s water quality. Regular water changes and the installation of a protein skimmer will help to keep on top of soluble wastes which although less apparent than solid waste, will also threaten a pond’s stability.

Yet in the natural, cycling eco-system in which koi are typically reared, the pond’s ecology and balance not only relies upon, but thrives on a steady stream of soluble and insoluble organic waste. As the natural stocking rates will be considerably lighter than an intensively stocked pond, the amount of food being processed and digested by koi will be reduced, making the levels of waste organic matter far more manageable for scavenging organisms. The sediment of a mature clay pond is a thriving industry of organisms, each existing alongside the next, yet adapted to a subtly different niche.

Upon harvesting a clay production pond, that is a mere 8 months old, thousands of invertebrates and insect larvae can be uncovered from the silt. This represents quite a living buffet for koi (and one of the factors that makes mud ponds ideal for rearing koi), yet so out of place in a highly filtered, re-circulating koi pond.

From tracing the flow of energy and food back through the chain, it is clear that these pond organisms are thriving as a result of the food that has entered the pond and been made available in its partially processed form by koi. The same balanced result would not have occurred if the food had simply been added to a koi-free pond. Koi perform a metered processing function that is fundamental to the life of a natural pond. Furthermore, as the cycle is allowed to develop, these scavenging organisms buried in the silt will eventually be rooted out by the inquisitive snout of a koi, themselves being digested, processed and excreted for their compatriots to utilise only hours later.

What do the processes of digestion and excretion mean for the koi keeper?

Our role as food provider is one of great responsibility, not only so that koi receive the correct nutrition, but also, so that the water quality can be retained.

Koi are never 100% efficient at digesting or absorbing their food so ultimately some food will be excreted (in both soluble and insoluble forms) back into the pond. This has implications for the long term health of the pond, being aware of the consequences of the accumulation of soluble and solid organic matter in a pond.

Inefficient digestion and excretion plays a positive role in a natural water body, but can cause real problems in an intensively stocked and fed koi pond.

The natural tendency is for a pond to accumulate organic matter, being processed by sedentary micro-organisms, that polish off any residual organic matter, becoming food items themselves. However, a bio filter should be protected for the accumulation of solid organic matter as the resultant blocking and clogging of biological media can cause problems in such heavily stocked ponds.

In summary, koi are just like any other vertebrate animal in that they depend on digestion to process food so they can absorb and transport it around their body. Other aquatic organisms also rely of the koi’s partial digestion and absorption of its diet by utilising any organic matter that is excreted. We must be aware of the possible lag between feeding and excretion and guard against the accumulation of organic material in the pond, – something that quite paradoxically, enables a natural clay pond to thrive.

Kill blanketweed and string algae.