Keep koi healthy in winter – 15 tips

Whether you stay in warmer climates or in colder regions, winter will most probably see a difference in day temperatures. In South Africa we have a wide variation of winter temperatures to deal with, but the good news is that koi fish will endure all our weather conditions. If you care about your koi fish, follow these tips to keep them and your pond in good condition in winter.

Pond management

The colder temperatures in winter do affect the koi’s behaviour in that he slows down or goes into a hibernation phase. Most of the time, when temperatures are lower than 8°C he will either swim slowly or lie at the bottom of the pond. It is for this reason that we need to do the following:

  1. Keep the pond clear of all falling leaves and keep debris from collecting on the bottom of the pond as this rotten material may cause harmful bacteria to attach themselves to the resting koi.
  2. Make sure that your pump basket is cleaned regularly as the falling leaves may cause blockage.
  3. When temperatures drop below 8°C switch off venturi and any waterfalls into the pond as this will make the water even colder.
  4. Remove any air stones.
  5. It is not recommended to add heated water to up the temperature as these fluctuations will negatively affect the koi and is also a waste of money.
  6. You can reduce your pump flow during this time, but do not switch your pump off for longer than two hours at a time, as this can affect your biological system.
  7. Make sure that your filtration system is clean from sludge and debris.

Koi feeding

  1. Gradually slow down your feeding as the temperature decreases and stop feeding altogether when the water falls below 4°C.
  2. A koi doesn’t have a stomach, he has a long intestine. So, during hibernation at these cold temperatures the food cannot be digested and can cause internal bacterial problems.
  3. When feeding and depending on temperature of the water, the food should be eaten within 30 minutes. It is normal to reduce feeding to once a day or even once in two days.
  4. The lower the temperature of the water, the less you need to feed your koi.
  5. While it is normal to feed koi wheat germ based foods during winter, I prefer to keep them on a mixed diet to assist their immune systems.

Koi health

  1. When the water temperature drops below 2°C, it is then critical for the koi as ice can form within the gill membrane (their lungs). In such cases I would suggest adding 10% new water to increase the water temperature slightly.
  2. I have seen koi ponds that are totally frozen on the top in winter with the koi resting at the bottom where the warmer water is. They are quite comfortable, so don’t panic.
  3. At very low temperatures I have even found koi who appear dead, but are in a very deep hibernation phase in order to conserve energy. In this case you will see gill movements, but not as regular as in warmer conditions, so just leave them alone.

In a nutshell

Some areas of the country are colder than others, and you may or may not see your koi hibernating. This is normal for them. But, if you suspect that there is something wrong, then rather consult your vet or a koi specialist.


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