Skip to content

Winter care

By the koi.wiki editorial team · Updated

Key facts: Winter koi care focuses on stopping digestion-triggering feeding once water cools below about 10 °C (50 °F), preventing the pond surface from freezing solid, and maintaining aeration so dissolved gases can escape beneath the ice.[1][2]
Koi are cold-blooded and cannot regulate their own body temperature, so their metabolism and immune function slow sharply as pond water cools[3][1]. According to the koi dealer Kodama Koi Farm, koi enter a dormant state below roughly 50 °F (10 °C), at which point feeding should stop to prevent digestive problems[2]. Wikipedia similarly notes that below 10 °C (50 °F) koi immune systems are very weak and digestion slows nearly to a halt, with appetite not returning until spring[1].Winter management mainly involves deciding whether to leave koi in the outdoor pond, move them to an indoor holding tank, or cover the pond, while ensuring the water does not freeze solid and that gas exchange continues through an open hole in the ice[3].

Feeding and water temperature in winter

Kodama Koi Farm states that the turning point for feeding decisions is around 65–68 °F, with feeding programs adjusted as temperatures fall through that range[3]. As autumn progresses, the same source recommends reducing feeding below 60 °F and stopping once koi become dormant below 50 °F, to avoid poor digestion that can prove fatal by spring[2][3]. Wikipedia likewise advises that feeding is not recommended once water drops below 10 °C (50 °F), since digestion has nearly stopped and koi may only nibble algae from the bottom[1]. Some specialty foods are marketed by dealers as digestible down to about 45 °F, according to Kodama Koi Farm[3]. A pond thermometer is recommended to track temperature and guide these decisions[3].

Ice cover and keeping an open hole

If a pond freezes over completely, gas exchange with the atmosphere stops while fish and decomposing organic matter continue to consume oxygen and produce waste gases underneath[5][3]. Wikipedia explains this as a general limnological phenomenon: ice and snow cover block light and air-water exchange, and when oxygen becomes badly depleted under ice, a “winter kill” of aerobic organisms can occur[5]. Kodama Koi Farm similarly recommends keeping a pond air pump running through winter specifically to maintain a hole in the ice, de-gas the pond, and prevent it from freezing over completely[3]. The same source states that if a koi becomes frozen in ice it will not survive, and recommends a heater in cold climates to prevent this[3].One hobbyist account cited by Kodama Koi Farm described running air pumps and a waterfall through a Colorado winter reaching −20 °F, with the waterfall freezing over in January–February but koi overwintering successfully on the pond bottom for over a decade without feeding[3]. Another keeper in Alberta, Canada, reported overwintering koi in a pond covered with an insulated frame, using a floating cattle-trough heater to hold water at 0–5 °C and a single pump for underwater flow, with no waterfall (to avoid over-cooling the water) and no winter feeding[6].

De-icers and pond heaters

Kodama Koi Farm recommends using pond heaters in winter (and shading or aeration in summer) to avoid temperature extremes, reduce fish stress, and help prevent disease[3]. The same source lists a titanium pond heater and controller, and a separate koi-specific pond heater, as examples of equipment used for this purpose[3]. See also the overview of heaters and heat exchangers for equipment specifications. Kodama Koi Farm also notes that pond salt is not a substitute for a heater: if a pond heater is in use, that is generally considered enough to keep koi functioning through winter, and salt is instead recommended mainly in quarantine or temporary tanks to boost disease resistance[6].Stopping waterfalls and pumps is also commonly advised: Kodama Koi Farm recommends stopping or bypassing waterfalls in winter because they expose water to cold air and chill the pond, and suggests stopping the main water pump and filter so moving water does not disturb hibernating koi, while draining any stopped filters so they do not hold stagnant water until spring[3].

Pond depth and design for overwintering

Pond depth affects winter survival. Wikipedia states that koi ponds in regions with harsher winters generally need a minimum depth of about 1.5 m (5 ft), compared with roughly a meter or more in milder, warm-summer climates[1]. Kodama Koi Farm similarly suggests aiming for about five feet of depth where ponds freeze, though it notes customers have had success with only three feet[3]. Texas A&M AgriLife Extension notes that most ornamental ponds are 18–24 inches deep overall but recommends including sections 3–5 feet deep specifically to resist winter freezes and to give fish a cool retreat in summer[7]. For general layout considerations, see pond design: size, depth and volume.

Covering the pond

Kodama Koi Farm outlines several covering options for winter, including building a greenhouse-style frame of PVC or wood over the pond covered in plastic sheeting, or laying a pool-style solar cover over the pond surface, weighted down with rocks, pipes, or lumber spaced about one to two feet apart[3]. In both cases, the source stresses keeping an air pump running underneath the cover to continue de-gassing the pond and to prevent the water from freezing over completely[3]. A pond cover is also described as useful for keeping leaves and debris out of the water during winter[3].

Moving koi indoors for winter

As an alternative to leaving koi in an outdoor pond, Kodama Koi Farm describes setting up an indoor holding tank, ideally in a garage, equipped with an established or establishable filter and a heater[3][4]. The source suggests keeping such a tank around 50–55 °F with light feeding of digestible foods through the season, or raising it to about 65 °F or higher for owners who want to feed more actively[4]. It cautions that if no indoor holding setup is available, the better course is to prepare the outdoor pond thoroughly instead, including heavy feeding through summer and fall and testing and treating the water for parasites before winter sets in[4]. For koi kept in indoor tanks or aquariums generally, the All Japan Nishikigoi Promotion Association notes that while nishikigoi can be kept at room temperature year-round, a heater and thermostat are advisable for insulation during winter months[8].

Health checks and parasite prevention before winter

Kodama Koi Farm recommends medicating the entire pond twice a year, once in spring and once in fall, to address parasites before koi become more vulnerable as water temperatures drop[3]. Wikipedia notes that water changes generally help reduce disease risk and keep koi from becoming stressed, and that proper oxygenation, pH stabilization, and off-gassing are particularly important to maintain over winter in small ponds[1]. If koi show signs of illness during the cold season, a fish veterinarian or qualified diagnosis is advisable rather than self-treatment; see diagnosis: scrapes, microscopes and fish vets and quarantine for related practices.

Common questions

At what temperature should I stop feeding koi for winter?
Kodama Koi Farm advises reducing feeding below about 60 °F and stopping once koi become dormant below roughly 50 °F (10 °C)[2][3]; Wikipedia similarly states that feeding is not recommended below 10 °C (50 °F) because digestion nearly stops[1].
Why do I need to keep a hole in the ice?
Wikipedia explains that under ice cover, gas exchange with the atmosphere stops while respiration continues, which can deplete oxygen and cause a winter kill[5]. Kodama Koi Farm recommends running an air pump through winter specifically to maintain an open hole and de-gas the pond[3].
Can koi survive if they freeze into the ice?
According to Kodama Koi Farm, a koi frozen into ice will not survive the winter, which is why de-icers, heaters, and aeration are used to keep the surface open[3].
How deep should my pond be for winter survival?
Kodama Koi Farm suggests aiming for about five feet in freezing climates, noting some ponds manage with three feet[3], while Wikipedia cites a minimum of about 1.5 m (5 ft) for harsher-winter regions[1]. Texas A&M AgriLife Extension recommends including sections 3–5 feet deep to resist winter freezes[7].
Should I use pond salt instead of a heater in winter?
Kodama Koi Farm states that if a pond heater is already in use, that is generally sufficient to keep koi through winter, and recommends reserving salt mainly for quarantine or temporary tanks rather than as a winter survival measure[6].

References

  1. ^ Koi (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Koi. Retrieved November 2026.
  2. ^ How Long Do Koi Fish Live? What is Lifespan & Growth?. Kodama Koi Farm (koi dealer). kodamakoifarm.com/how-long-do-koi-fish-live/. Retrieved November 2026.
  3. ^ Koi Pond in Winter: Best Tips for Winter Koi Pond Preparation. Kodama Koi Farm (koi dealer). kodamakoifarm.com/winter-koi-pond-preparation-survival/. Retrieved November 2026.
  4. ^ How Can I Safely Transport My Koi Fish?. Kodama Koi Farm (koi dealer). kodamakoifarm.com/how-can-i-safely-transport-koi-fish/. Retrieved November 2026.
  5. ^ Hypoxia (environmental) (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Hypoxia_(environmental). Retrieved November 2026.
  6. ^ Benefits of Salt in Koi Pond & Tips for Safely Using It!. Kodama Koi Farm (koi dealer). kodamakoifarm.com/benefits-salt-koi-pond-why-should-use-it/. Retrieved November 2026.
  7. ^ Ornamental Fish Ponds and Water Gardens in Texas (Texas A&M AgriLife Extension). Texas A&M AgriLife Extension. extension.rwfm.tamu.edu/wp-content/uploads/sites/8/2013/09/Ornament…. Retrieved November 2026.
  8. ^ How to keep Nishikigoi - All Japan Nishikigoi Promotion Association. All Japan Nishikigoi Promotion Association. jnpa.info/en/nishikigoi/breeding/how-to/. Retrieved November 2026.