Do Koi Fish Sleep? + How & Why

Are you wondering whether your koi fish is sick or just sleeping? Of course, Koi fish can sleep. But not like most animals or humans. Koi fish lack an eyelid. Therefore, they cannot close their eyes. That’s why when a koi fish is not moving, you might think it is sick or dead. Only check for illness if it stays in the same position for more than 8 hours.

Koi fish sleep usually at night but can do so during the day if bugs are active at night. In that case, they feed at night then rest during the day. They normally sleep as a group and will always look for safe areas within the pond for rest and sleep, from late evening to early morning, and with their eyes open.

They are active fish. Therefore, they need sleep to improve their physical health. Extremely low or high temperatures can interfere with koi fish movement. But the fish will grow to 2 or 3 feet and live long if you take good care of it. In this article, you will learn more about how koi fish sleeps, when, and how temperature affects their sleep.

Group of Koi Fish

Do koi fish sleep?

Yes. Koi fish sleep at night but in quite an unusual way. They do not need an eight-hour sleep cycle like humans. What they need is constant rest.

Koi fish can search for comfortable and safe resting areas within the fish tank. It prefers to rest and take a nap at the bottom of the tank to protect itself from predators such as hawks, owls, and raccoons. 

Besides, the water at the pond’s bottom is cool, and it fastens the koi fish’s resting process.

How do koi fish sleep?

Koi fish sleep with their eyes wide open. You may think that they are awake, but they are resting. They choose to rest at the bottom or the middle of the pond. Usually, the fish will sleep in groups to be safe from snakes, hawks, and owls.

The sleeping position of the koi is upright. They do not sleep on their back. So, your Koi might be sick or dead if it’s sleeping on its side. They will rest by remaining afloat in some areas of the pond. 

You should learn your fish’s sleeping habits and behaviors to know when it is in good health. While shrimps may play dead when resting or molting, most fish sleep in many different ways.

When do koi fish sleep?

Koi fish sleep at night. Usually between late evenings and early mornings. However, some koi fish will occasionally wake up to graze even at night when there are bugs around. Those that ate something at night may develop a daytime sleep cycle.

Tip: Do not disturb the fish during its sleep unless it has not moved for the last eight hours.

Does temperature affect koi fish sleep?

Temperature is the most essential determinant of koi fish sleep. During cold months, your koi fish will resort to hibernation. To hibernate, their metabolism reduces, lowering their temperature and food intake. 

It is a survival mechanism that helps your kois conserve energy. It also allows them to survive the cold seasons with less food.

When in hibernation, koi fish will stay afloat in the pond. You may think that the fish are not moving or that they are dead. But a closer look reveals slow-moving fins. You need clear pond water to observe that.

Therefore, your fish needs a lot of rest, particularly during winter. Do not disturb the fish during its sleep unless it has not moved for the last 8 hours. A good rest will also determin how fast a koi grows since it reduces stress.

Install a de-icer in your fish tank if the weather is freezing. A de-icer will help reduce ice formation and protect your koi fish from freezing to death.

Also, ensure that your pond is adequately aerated. In that case, the pond will have enough oxygen to support your koi fish even when they are hibernating. Failure to install an aeration system may lead to the death of your koi fish in extreme temperatures. Such systems make koi ponds cost a good amount of money to set up.

Koi fish activity level depends on the pond or aquarium temperature. Once the water temperature is back to between 65 and 75 degrees Fahrenheit, the fish’s metabolism will increase, thereby increasing your koi’s activity level. 

Active koi fish swim and eat a lot, which means they excrete a lot of waste as well. Therefore, clean your koi’s habitat more frequently during the hot summer months. In short, a water temperature below 50°F is too cold and above 80°F is too hot for koi fish.

Do koi fish sleep together?

A large group of koi fishes can sleep together at the same time. They prefer it that way for security purposes. A group sleep improves their reaction to predators such as snakes, hawks, and owls. The more koi fish in your fish tank, the more likely they will sleep together.

However, fewer koi fish in captivity can sleep alone due to the improved security and care from their owners. For instance, I have seen some of my koi fish sleeping alone at the bottom of the tank and hiding spots during the day and at night. 

Is my koi asleep or sick?

If your koi fish is unresponsive but lying upright, it may just be sleeping and not sick. But only if it’s been less than 8 hours since the fish became unresponsive. Check the behavior of your koi fish to know whether they are sick or asleep.

Healthy koi fish are quite active. They need sleep to improve their physical health. However, extremely low or high temperatures can interfere with koi fish movement. It can lower their metabolism and cause Koi Sleepy Disease.

Sick koi fishSleeping koi fish
Unresponsive Responds to touch and disturbance
Lying on the side or backStays upright
Sunken eyesHealthy and normal eyes
Lethargic Active in playing and eating
Swollen gillsNormal and healthy gills
Not eatingEats normally in amount and diet

Not every time the fish is dormant it will be asleep. Sometimes it could be sick. Do you know about Koi Sleepy Disease (KSD)? It is also known as Carp Edema Virus Disease. The disease is rampant in juvenile koi fish during rainy seasons. 

Koi Sleepy Disease can also be caused by the stress subjected to the fish during handling or from loud noises such as when drilling the pond to install aeration systems. If KSD is not treated quickly, you can lose over 80% of your koi fish population.

Some signs of a sick koi fish include:

  • Unresponsiveness
  • Lying on their sides or their back
  • Sunken eyes
  • Lethargy
  • Swollen gills
  • Not eating

Always request test results of Koi Sleepy Disease when buying a new koi fish.

The good news is that the Koi Sleepy Disease is treatable and preventable. Ensure that you clean your ponds regularly to prevent CEVD. Avoid any event that can stress your koi, and ensure that you feed it a nutritious diet.

Secondly, use pure non-iodized salt to control the disease in your fish tank. Apply 0.3% – 0.6% salt in the quarantine pond.  

Additionally, remove koi fish that die of sleepy disease out of the fish tank immediately to reduce the transmission rate to healthy koi. If the fish tank is overcrowded, reduce the fish’s population so they can have enough space to swim and reproduce.

Nevertheless, the fish will grow to 2 or 3 feet and live long if you take good care of it. 


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