Guide to Koi Behaviors – What Do They Mean?

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Group of koi
Koi behavior varies greatly depending on the current season and sexual maturity. Appie Verschoor / CC BY-SA 2.0

Freshwater fish enthusiasts can spend hours watching and fawning over koi, and it’s no mystery why. These expressive fish have distinct personalities and are constantly on the move as they explore every nook and cranny of a pond. At one moment, they may seem relaxed and fully content. Suddenly, they can begin thrashing about and bumping heads, glistening under the warmth of the summer sun. It’s this flurry of activity that can keep spectators on their feet and unable to step away from the pond’s edge.

Normally, koi behaviors are determined by a seasonally changing set of physiological drives. The way they move in winter can be starkly different from how readily they dive and feed in summer. Males and females, once they become sexually mature, may behave differently from one another too. The most important behaviors to look out for are those that may be associated with stress, which is often the result of overcrowding, suboptimal pond conditions, or diseases.

It is advisable for koi owners to thoroughly familiarize themselves with koi behavior. Constant observation of these fish will allow you to quickly differentiate between normal and abnormal activity. It will also help you respond in a timely manner to potential pond problems that need to be addressed. You’ll be surprised to find how revealing a koi’s behavior can be!

Are My Koi Behaving Normally?

Girl hand-feeding koi
Koi have long-term memories and can be trained to remember faces and be fed by hand! Kelly Verdeck / CC BY-ND 2.0

As domesticated fish, koi are often compared to dogs! They may not be able to respond to specific words or travel with you, but they can reward your presence with a similar measure of elation. Healthy koi are generally happy and gregarious fish that will rarely shy away from attention. They may even crave tactile interactions with humans. Furthermore, they have long-term memories and can be trained to remember faces, know their own names, and feed from a hand.

Koi behavior is largely influenced by the weather and the quality of your pond water. Healthy koi may appear to swim lazily around the pond. They will readily gather around you during feeding time, pushing their heads through the surface of the water. If temperatures are mild to warm, they tend to be more energetic. Optimal dissolved oxygen and nutrient concentrations in your pond water should leave them feeling comfortable and relaxed all throughout the day.

At night, koi may appear to gather together as they rest. If food is available, however, they may jump at the chance to forage for and catch prey. They are usually less active at night, especially if they are routinely fed during the daytime. A good way to tell if one koi is acting normally is by comparing its behavior to that of your other fish and by taking note of how each one behaves on a daily basis. If something seems out of the ordinary, you will need to pay closer attention.

Signs of Stress

Stress is readily observable in koi, which are highly sensitive to their surroundings. Spikes in toxic nutrient levels, low oxygen conditions, and extreme highs in temperature may cause your entire community of fish to act out. Unfavorable conditions may seriously harm some individuals, compromising their immune response. The behavioral signs listed below are usually indicative of physiological stress or damage.

1) Flashing and jumping

Koi flashing
If you see your koi repeatedly jumping for no reason, they may be sick or stressed. Andreas Koppe / CC BY 2.0

Flashing may refer to an unusual set of koi behaviors. A “flashing” fish may be dashing around the pond in an odd manner, swimming sideways or upside-down, repeatedly jumping for no apparent reason, or rubbing its skin on rough surfaces. These are usually done in response to an ectoparasite or due to poor oxygen conditions and spikes in ammonia concentrations. Sick or stressed fish can easily become delirious, which explains why they would exhibit repetitive flashing behavior.

2) Gasping for air

Koi at water surface
If most of your koi are gasping for air, it’s most likely a sign of low dissolved oxygen levels. xflickrx / CC BY-SA 2.0

Low dissolved oxygen levels can cause koi to rise to the surface and repeatedly gasp for air. Overcrowded ponds or those with faulty filtration units tend to have frequent oxygen problems that may eventually become lethal for your koi. If this behavior is observed in just one fish, and all other individuals seem to be okay, it may simply be due to curiosity or hunger. If most fish begin to gasp for air, you must act immediately. Provide supplementary aeration and conduct a water change if needed.

3) Lethargy or immobility at the pond bottom

Koi at pond bottom
Koi that are lethargic or immobile & stay at the pond bottom all day are most likely sick. Stan Shebs / CC BY-SA 2.5

Though it is normal for koi to be content to stay in one spot, they will rarely do so for the duration of an entire day, especially when ambient conditions are favorable. They need to move to feed, stretch their muscles, and socialize with other fish. Koi that remain lethargic or immobile tend to stay close to the pond bottom and may even appear to be perched on the ground. If they behave in this manner for extended periods of time, even throughout feeding sessions or when they are gently prodded, they are likely very sick. Isolate the immobile fish and attend to your pond’s water parameters right away.

4) Skittishness


Koi that have just been transferred from one water feature to another may be extremely skittish for a few hours to days. The same goes for imported koi that have just been introduced into the quarantine setup. Their skittishness will naturally go away as the fish adjusts to its new surroundings. Skittishness may also occur due to the presence of a predator.

5) Hiding or self-isolation

Lone koi
As koi are generally social fish, there may be something wrong if they are isolating themselves for long periods of time. John Lodder / CC BY 2.0

Koi that prefer to remain hidden or self-isolated for extended periods of time may be depressed, wary of predators, or sluggish due to ill health. As they are remarkably social fish, happy koi will rarely prefer to be isolated. Self-isolating behavior may be normal for fish that have just been introduced into the pond or older ones that are taken aback by new changes. If your pond water is in perfect condition and all other fish appear healthy and active, prolonged isolation may simply be due to the fish’s personality or there may be an underlying source of stress.

Spawning Behavior

Koi spawning behavior
When a female koi is about to release eggs, male koi will incessantly push and prod against her. Andy Rogers / CC BY-SA 2.0

The spawning period is a remarkably exciting time for koi. As the female is about to release her eggs, males become increasingly drawn to her and may exhibit vigorous behavior. The female will usually be pushed close to vegetative features which can act as anchor points for the fertilized eggs. Up until all eggs are released, the males will incessantly push and prod the female.

The actual spawning event usually begins very early in the morning, even before sunrise. Eggs may be released in batches all throughout the morning or even into the afternoon and evening. You can expect both male and female spawners to be extremely spent after the event. They may appear sluggish for the next couple of days as they recover. If your water conditions are optimized, recovery rates will be much quicker. During these days, it is also normal for the koi to consume a large portion of the released eggs. They may deplete almost all of the eggs, which usually number in the hundreds of thousands per gravid female. Eventually, they will eat a fair amount of the hatched young.

Surface Feeding & Foraging Behavior

Koi feeding
Wild koi tend to be bottom feeders, whereas ornamental koi feed at the surface of the pond. Kristin “Shoe” Shoemaker / CC BY-ND 2.0

In the wild, naturalized koi populations and escapees tend to be bottom instead of surface feeders. As omnivores, they feed on a wide variety of food types and may be quite opportunistic when it comes to their preferences. In contrast, koi in ornamental ponds are trained to take fish feeds at specific times during the day. This causes them to densely gather close to the source of food, mouths agape at the surface.

Ornamental koi can even be trained to take food directly from a human hand. Though they have teeth, they are extremely unlikely to bite. They are avid eaters that will consume a full portion of food in just a few minutes. Pond koi may also attempt to feed on natural food sources, such as tadpoles, aquatic insects, and the fleshy shoots of plants. These food items encourage them to move toward and explore the pond bottom. If they are sufficiently fed, they may do so out of curiosity instead of genuine hunger.

Koi Winter vs Summer Behavior

Koi pond in winter
Koi enter a state of torpor in the winter, meaning that they hardly eat and appear immobile for long periods of time. Scottb211 / CC BY 2.0

As temperatures begin to significantly drop toward winter, koi will naturally retreat toward deeper parts of the pond. They do so because temperatures are normally warmer closer to the pond bottom, while surface temperatures continue to cool. As they enter a hibernation mode called ‘torpor’, they will rarely take food and may even appear immobile for lengthy periods of time. This is the only time of the year when this behavior is considered completely normal, particularly if temperatures dip to below zero.

Your koi will begin to perk up as soon as temperatures increase. Their metabolic demand will gradually increase, causing them to search for food. In spring, you’ll find them gathering closer to the surface and stretching their fins. Their feeding capacity is highest when water temperatures reach 16 – 18.5˚C (61 – 65˚F). This is also when they are most active and likely to spawn. Energy levels become significantly reduced, once more, in fall.

Nocturnal vs Diurnal Behavior

Koi pond
Koi like to explore their surroundings in loosely-packed schools during the daytime. ElCapitanBSC / CC BY-SA 2.0

During the day, wild koi or those in large ponds may slowly explore their surroundings. They do so in loosely-packed schools. They are generally peaceful fish that seldom exhibit aggression toward other koi and pond animals. Those in ornamental ponds are not afraid to expose themselves fully, allowing their forms and colors to be admired by spectators. In ponds that are frequently visited by wild animals and potential predators, koi are more wary and skittish.

The nocturnal behavior of koi is largely influenced by their feeding regimen. If they are fed regularly during daylight hours, they will need to rest at night. They may move around slowly but will be more restful compared to how they are during daylight hours. If they must hunt for food, they are more likely to be nocturnal feeders. Many of their protein-rich food preferences, such as insects and larvae, may be more active at night. Nocturnally active koi must find time to rest during the day.

Can Koi Exhibit Aggression?

Koi with goldfish
Koi are known for being calm and friendly, though they can display aggressive behavior towards fancy goldfish! Kamillo Kluth / CC BY 2.0

Koi may exhibit aggressive behavior if they are stressed, while breeding, or in the presence of other fish species. An unsuitable diet, aggressive pond mates, and poor pond conditions can force them to enter survival mode and feel the need to defend themselves.

Koi are also known for being particularly aggressive toward one type of ornamental fish – fancy goldfish. Oddly, they have the tendency to nip and chase these smaller fish. That being said, aggression is generally rare as koi are calm and friendly by nature. It can easily be avoided by maintaining favorable pond conditions, feeding them with high-quality feeds at the right frequencies, and housing them with friendly species.

On occasion, a koi may inadvertently or intentionally eat or bite smaller fish. They should be housed with companions that are at least several inches larger than their mouths to prevent this from occurring. Good pond mates include orfes, plecos, and larger types of goldfish. Note that overstocking the pond can lead to aggression-related accidents as the koi must compete for space and food. They become increasingly irritable and hostile as pond conditions are worsened.

Nonetheless, there’s a reason why koi are the most popular ornamental fish. If they are happy and healthy, they’ll reward you with a positive temperament and ease of care for many years. Without fail, responsible pond ownership should bring out their best behavior!

Chris G
About the author

Chris G

Pond consultant and long-time hobbyist who enjoys writing in his spare time and sharing knowledge with other passionate pond owners. Experienced with pond installation, fish stocking, water quality testing, algae control and the troubleshooting of day-to-day pond related problems.

Read more about Pond Informer.

1 thought on “Guide to Koi Behaviors – What Do They Mean?”

  1. We have one female that will chase a male for about 2 hours around the tank.
    Why is she doing this? Everything in the tank is kosher(good).


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