estanque koi invierno

Koi in winter

Koi in winter behave differently. Here we talk about what is normal and what is not normal. If you have any further doubts after reading this article, please contact us. 

Koi belong to the carp family (cyprinus carpio) and are cold water fish, that means that their body temperature depends on the temperature of their surroundings. They can live in waters with temperatures between 2º to 35º.

If the temperature change is carried out progressively, there is no problem. But a change of temperature of more than 5º in a same day can cause serious problems. In large ponds, especially with sufficient depth, temperature changes are less stressful, while in small, flat ponds, koi are exposed to living the changes more drastically.

Koi can easily survive 2 months to 2º.

Taking into account that the temperature of the body of the koi usually varies half a degree with the temperature of its environment, it is understandable that the metabolism of the fish is very reduced during the winter season.

Generally the koi in winter are usually “parked” at the bottom of the pond.

Koi are hibernating. 

At these water temperatures it is imperative to leave them in peace.


Because they are with a reduced metabolism resting in the pond and if suddenly you approach and make some sudden movement you frighten them and cause them to flee intuitively. Then they have to make a huge effort that causes them a lot of stress.

When the metabolism decreases, the defences also decrease and the koi are more susceptible to diseases such as Saprolegnia (a type of fungus that can live between 3º-33º) and to lie sidewards (usually a problem of the swim bladder).

Attention: Make sure the surface of the pond is not completely frozen. You need at least a good open hole so that toxic gases can escape and oxygen can enter. NEVER – I repeat NEVER – break the ice in case the surface is frozen. The reason is explained above (not to scare). You can get a hole by pouring hot water. To maintain it you can use a heater or special elements of styrofoam.

Do you feed koi in winter?

I just got a call from a client: “Thekla, my koi do not eat. For 2 weeks now they no longer eat. The water temperature is 8º. How long can they be without food?”

koi en invierno

Koi Hacienda: Do you feed koi in winter?

Nature is very wise. Normally, koi stop “asking for food” at around 8º of water temperature. From this temperature on, it is advisable not to feed at all. They can easily survive 2 months without eating (or even more).

Between 8º and 12º we recommend to give them special winter food. This is prepared for easier digestion and thus does not cause too much work for the metabolism.

Can I buy koi in winter?

You can always buy koi 😀

The most accurate question should be: Can I introduce new koi in winter?

When the water temperature of your pond is 12º and less, we will not send the koi. The koi has to withstand the stress of the trip (although we try to put it as comfortable as possible, it will always be stressful) and on top low temperature, would be too much for the organism of the koi. Introducing it when there are high temperatures is easier, but introducing it with cold is very delicate and you have to calculate a long time of adaptation (30-40 minutes).

If a koi is introduced into too cold water, 2 things can happen:

  • the koi lives a heart attack and dies right away
  • the koi lives a “shock” whose consequences may appear days later: behave “weird”, swim “crazy”, float but still breathe and finally die

Good to know

  • Our koi come from the Niigata area of ​​Japan, where year after year there is a hard winter with a lot of cold and lots of snow.
  • A koi kept only in hot water DOES NOT grow more than a koi that lives its rest period in winter.
  • An appropriate rest period is 2 months at 8 degrees.
  • The rest period is absolutely healthy: it reduces the risk of accumulation of eggs, tumors and other metabolic diseases.
  • In areas where the water temperature does not reach 8º, it is advisable to take 2 weeks in the coldest period of the year and put them to fast (for the reason explained above).

And then?

When the temperature rises again, the problems in the pond explode.


Because the metabolism of the koi has to “light up” little by little. While the metabolism of parasites and bacteria reacts much faster. So the “attackers” are ready while our koi begin to “wake up.”

For your koi to survive the winter well and be strong in spring, we recommend the natural product Malamix17  with 17 natural ingredients to increase the immune system of the koi itself.

If you have any questions, please do not hesitate to contact us: