Feeding Koi by Water Temperature

Koi metabolism follows water temperature. Published feeding thresholds, why wheat-germ-type diets suit cool water, protein and frequency by season, and the risks of overfeeding.

Metabolism follows water temperature

Fish are poikilothermic: the Merck Veterinary Manual states that all their physiological processes are strongly influenced by water temperature, including antibody production, which depends on rising temperature to work efficiently. Practical Fishkeeping gives the working rule that a fish's metabolic rate roughly halves for every 10 °C drop in water temperature. For koi, Wikipedia gives 15–25 °C as the range in which they do best, and a peer-reviewed review in PubMed Central places the optimum at 23–25 °C, with 15–35 °C acceptable and conditions below 12 °C or above 35 °C critical. In winter koi digestion slows almost to a halt and they eat very little; Wikipedia adds that their immune system is severely compromised below 10 °C. Because air and water temperatures diverge, Practical Fishkeeping advises deciding what to feed from a thermometer in the pond, not from the weather.

Published feeding thresholds

16 °C and above: a high-protein staple diet (Practical Fishkeeping).
10–15 °C: mix the high-protein diet with a wheat-germ-based food (Practical Fishkeeping). Tropical Fish Hobbyist gives the same transition at 60 °F (about 15.5 °C): switch to a low-protein food and begin tapering the amount.
Below 10 °C: wheat-germ-based food only, in small amounts, until the fish stop taking it (Practical Fishkeeping). TFH advises stopping altogether once the water falls below 50 °F (10 °C) and stays there, and Wikipedia states that feeding is not recommended below 10 °C.
Down to about 5 °C wheat-germ food remains digestible (Practical Fishkeeping), but fish that no longer show interest should not be offered food.

Why wheat-germ-type diets in cool water

Practical Fishkeeping explains that a slower metabolism means a less efficient digestive tract, so the fish need a food that supplies energy but is easily digested; wheat-germ-based formulations are the category used for this purpose. For the warm season, the PubMed Central review gives the typical composition of a koi maintenance feed as 31–33 % protein, 4–6 % fat, 3–5 % fibre and 9–10 % moisture. Koi are omnivorous bottom feeders (Wikipedia) and the Merck Veterinary Manual lists pelleted foods formulated for koi and goldfish as the base diet, supplemented with natural items such as shrimp pieces and plant material, plus insects that fall into an outdoor pond. Wikipedia notes koi will also take peas, lettuce and watermelon.

Frequency and amount by season

Spring: reverse the autumn progression as the water warms (Practical Fishkeeping). SRAC notes that a biofilter that went unfed over winter has lost capacity, so feed rates should rise slowly while bacteria recover. TFH advises leaving waterfalls off until the water reaches 60 °F (16 °C) to avoid cold shock.
Summer: metabolism and activity peak, so the high-protein diet applies. The PubMed Central review recommends feeding twice a day; TFH describes two to three small feeds a day in the active season, with all food eaten. Practical Fishkeeping advises removing anything uneaten after about 20 minutes.
Autumn: taper the amount and move to the wheat-germ-type food below 15–16 °C. TFH stresses that the goal is to prevent weight loss, not to fatten the fish, before a semi-dormancy that lasts four or five months.
Winter: stop feeding when the water stays below 10 °C. Merck adds that any food offered in cold weather should be reduced because the fish are less active and need less.

Overfeeding risks

The University of Florida IFAS states that the more a fish is fed, the more ammonia it produces, and un-ionised ammonia of only 0.05 mg/L can damage fish tissue. Uneaten food decomposes and consumes oxygen, and UF/IFAS lists excess fish feed among the organic loads behind oxygen depletion; warm water already holds less oxygen (7.4 mg/L at saturation at 90 °F/32 °C against 11.9 mg/L at 45 °F/7 °C). Tropical Fish Hobbyist identifies overfeeding and overstocking, especially while the biological filter is still maturing, as the most common cause of water-quality problems in koi ponds, and treats food left uneaten as a sign of a management problem. The Merck Veterinary Manual advises checking regularly whether fish are too fat or too thin and not letting pellets dissolve before they are eaten. If ammonia or nitrite is detected, UF/IFAS states that reducing or stopping feed for 1–3 days does not normally harm fish.