The danger of overfeeding koi

On the question as to how often koi should be fed, enthusiasts and breeders are usually led (and often misled) by the “feed them as much as they will consume within five minutes” rule. This recommendation may however be ill advice when followed rigidly since there is no fixed guideline for all pond conditions:

Overfeeding of koi is a common practice. Because feeding evokes a response from their koi, enthusiasts often overfeed them. Although small mistakes in overfeeding may have no visible effects when water conditions are at optimum, larger mistakes may take their toll - so also constantly repeated ones that will leave their mark in some way when environment conditions change.

A sudden rise in oxygen consumption normally follows feeding due to the increased supply of nutrients for body metabolism (the bigger this consequent increase in metabolic rate, the more oxygen is required). Therefore, the more food the koi will consume, the bigger this demand for oxygen. Since the oxygen content of the water is restricted by external factors such as temperature, oxygen deficiencies are sometimes experienced when the “5-minute rule” is followed rigidly. Caution should always be taken that the feed-associated increased oxygen demand should never exceed the oxygen-carrying capacity of the pond water. A rise in water temperature will decrease the oxygen sustainability of the water and consequently oxygen available to the koi. Neglecting this reduced oxygen supply during feeding puts unnecessary stress on the koi and often leave them gasping for air close to the water surface (metabolism is increased at high temperatures, and oxygen quickly becomes a restricting factor in the maintenance of body functions).

Extreme oxygen deficiencies are especially experienced in plant- and algae rich waters during periods of bad light-intensity. Because plants and algae photosynthesize during the day – producing oxygen by day and consuming it by night – significant fluctuations in dissolved oxygen levels are experienced constantly. In algae-rich water, feeding should therefore not take place too early in the morning or in the evenings since feeding will cause a rise in metabolism and consequently in oxygen consumption (beware even of full feedings on rainy days or during long periods of cloudiness when limited photosynthesis will take place). The situation can be relieved during these periods by effective aeration of the water to prevent these excessive oxygen fluctuations. In ponds with optimally functioning biofilters and proper aeration, koi can normally safely be fed ad libitum (until it is satisfied) without any problems.

Additionally, instead of distributing a large quantity of food once off, food should be presented in more, smaller portions. Food intake should be spread evenly throughout the day to ensure that oxygen requirement peaks are likewise stretched over a longer period of time. Feeding koi this way will ensure that peak oxygen demand of the koi will seldom suddenly exceed the oxygen availability level of the pond water

Furthermore, oxygen-problems are sometimes amplified when homemade koi foods with poor water stability are fed. These foods normally disintegrate very rapidly in the stomach of the koi as well as in the water, causing a quick release of nutrients to the body and water and increasing oxygen demands suddenly in water with already limited oxygen availability. Extruded (low density, puffed) pellets, on the other hand, are more stable. It doesn’t break down that easily and the stomach is emptied at a slower rate to the gut where absorption takes place, which means oxygen requirements will increase at a slower rate with consequently less discomfort and stress to the koi. The slower emptying of the stomach distributes the oxygen demand over a longer period and prevents it from reaching critical peak levels, like is the case with some homemade foods.

Finally, the ability of koi enthusiasts to judge when enough food is enough is only learned through experience. It is important to always guard against over-consumption especially if the food is very palatable. “Little and often” is a good rule to follow, even if it means adopting a “2 minute“ rule. In this way the koi, while receiving enough food, are kept interested and will soon tend to hand-feed. Always remove uneaten food from the water when the koi loose interest in feeding to reduce the unwanted effect of nutrient leaching on water quality. When water temperatures drop significantly, the food intake of the koi will decrease accordingly because of its cold-blooded nature (its metabolism is influenced by its body temperature, and the body temperature by the temperature of the environment). It is therefore important to feed them sparingly during winter. When water temperatures drop below 5°C feeding becomes mostly unnecessary, as they will normally not eat.

The extent, to which the “5-minute koi-feeding rule” should therefore be followed, depends on the relevant situation. In certain cases, like poor water quality and/or low oxygen levels these recommendations are not strictly applicable. It remains important to monitor the reaction of the koi on changing environmental and water quality conditions in order to make sensible decisions concerning the feeding management.

Lourens de Wet