Koi are a type of fish that are diurnal, meaning they are most active during the day and rest at night. When resting, their metabolism slows down, they become less responsive to their environment, and one might think they are sick or dead.

However, koi fish do not have eyelids and cannot close their eyes while sleeping. They remain still at the middle or bottom of the pond, preferably in hidden areas where predators cannot spot them.

Koi fish sleeping

How do koi fish sleep?

Koi fish sleep with their eyes wide open, and you may think they are awake. They choose to rest at the bottom or the middle of the pond. Usually, the fish will sleep in groups to feel safe from intruders such as snakes, hawks, and owls.

While shrimps may play dead when resting or molting, koi floats upright when sleeping or hibernating. The fish might be severely sick if they sleep on their back or side.

When do koi fish sleep?

Koi fish sleep at night, usually between late evenings and early mornings. However, some koi fish occasionally wake up to feed on active bugs at night. This may force them to develop a daytime sleep cycle.

Koi’s movements can also be affected by temperature changes. During cold months, your koi fish will resort to hibernation. In this state, their metabolism reduces, which also reduces their food intake. This also allows koi fish to survive the cold seasons.

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

This means your fish needs a lot of rest, particularly during winter. Do not disturb it when sleeping unless it has not moved for the last 8 hours. A good rest will also determine how fast a koi grows since it reduces stress.

Note: Water temperature below 50°F is too cold, and above 80°F is too hot for koi fish.

What you need to do

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 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.

Once the water temperature is back to between 65 and 75 degrees Fahrenheit, the fish’s metabolism will increase, increasing your koi’s activity level. 

Active koi fish swim and eat a lot, which means they also excrete a lot of waste. Therefore, clean your koi’s habitat more frequently during spring and summer.

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’s sleep improves their reaction to predators such as snakes, hawks, and owls. This behavior is replicated in gourami fish which may swim on their sides or appear inactive when sleeping.

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.

Sleepy vs. sick koi

If your koi fish is unresponsive but lying upright, it may 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.

Here are the differences between a sick and sleeping koi fish.

Sick koi fishSleeping koi fish
Unresponsive to touchResponds to touch and disturbance.
Lying on the side or back.Stays upright in the water.
Has sunken eyes.Healthy and normal eyes.
Becomes lethargic.Active in playing and eating.
Swollen gills.Normal and healthy gills.
Not eating as needed.Eats normally in amount and diet.

Carp edema virus disease (CEVD) is a concerning problem for koi. The disease can cause high mortality rates in wild and cultured varieties of koi.

This disease is also referred to as “koi sleepy disease” (KSD) because infected fish become lethargic and unresponsive, explains the University of Florida IFAS Extension. It is rampant in juvenile koi fish, particularly 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.

In addition to constant unresponsiveness, sick fish may exhibit erosive or hemorrhagic skin lesions with swelling of the underlying tissues. It’s important to have your fish tested for the best treatment.

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 you feed it a nutritious diet.

Additionally, salting a koi pond can help control the disease. Apply 0.3% – 0.6% non-iodized salt in the quarantine pond.  

Remove any diseased koi fish from 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.


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