How to Stop Koi Fish Bullying (4 Steps)

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koi can become aggressive if overstocked
Although koi are usually gentle fish, they can become aggressive due to poor conditions, diet or unsuitable pond mates. Public domain.

You wouldn’t place a grizzly bear and a panda bear in the same space together, would you? One is known for its aggressive behavior, and the other just wants to eat bamboo and roll around in the grass all day. Likewise, different species of fish of course have different temperaments. Even fish of the same species may sometimes not get along with one another, as their personalities also play a role in overall behavior and temperament.

Though koi are typically gentle and easygoing fish, they can at times be aggressive and may bully other fish depending on the circumstances. Here we will cover the main circumstances under which koi can become aggressive, and what to do about them!

What Factors Make Koi Carp Aggressive?

1) General Temperament

some koi are naturally loners and like their space
Some koi are just grumpy by nature and prefer to be alone. Public domain.

As mentioned above, personality can factor into whether or not a koi is aggressive. If you notice that your koi seems to be naturally aggressive for no evident reason, it’s possible that you simply have an aggressive koi. Fish with more aggressive personalities, when they’re not chasing others around the pond, will often fight for food and be the first on the surface when food is made available.

2) Breeding & Spawning

breeding causes male koi to become aggressive
Breeding will cause males to become more aggressive than usual. Public domain.

During spawning, males can of course become hostile toward other males as they compete to fertilize the eggs released by females. This typically involves nipping at each other or maybe shoving each other around a bit. When males are pursuing females, this can be mistaken as aggression as they chase the females and may push them into things. This is simply to help the female release her eggs so that they can be fertilized, and most of the time is not actually bullying or attacking. Some injuries may occur to fish of both genders during spawning, so making certain that water quality is excellent will help them to heal quickly.

 3) Lack of Space & Overstocking 

too many koi in a pond causes stress and aggression
Over-stocked ponds will cause all sorts of issues, including heightened aggression. Photo by frank green on Unsplash

This one is pretty straight forward – if there’s not enough space in your pond, violent behavior is likely to ensue as fish compete for space, food, and shelter. In addition to potentially leading to fighting, too many fish and too little space also means that there is likely a nutrient overload in your pond, which in turn enables illness and disease as well as potential eutrophication and algal blooms. It’s important to never over-stock ponds, and the more space you can provide your koi, the healthier, happier, and less aggressive they will be!

4) Poor Quality Food & Nutrition 

Proper nutrition lessens koi aggression
Feeding the right amount and the right foods will keep koi happy. Public domain.

Just as with space, not having enough food is cause for your fish to not get along and perhaps become aggressive in order to obtain the nutrition that they need. Make sure that you are not only feeding your koi enough, but are feeding them the right types of food as well so that all of their dietary needs are met. A high quality pellet feed which contains a large amount of protein, healthy fats, carbohydrates, as well as vitamins and minerals is essential to the long term health and happiness of carp. You can check here for our recommendations of top quality carp feeds.

5) Illness or Injury

A sunburned koi that is more likely to be aggressive due to discomfort
A sick or injured koi may be aggressive itself, or bullied by healthier fish. Public domain.

Just as when we’re not feeling well, when a koi is sick or otherwise injured it can become irritable. This depends on the circumstances as well as the fish itself – some may lash out due to an instinctual drive to protect themselves from being viewed as the “weakest link” while they’re ill, while in other cases the illness itself may alter their physical and cognitive state in such a way that they don’t act like themselves. Koi could become restless, melancholic, sluggish, or hostile until their health is restored.

6) Heightened Stress

A predatory heron causes stress and potential aggression in koi and other fish
Predators, such as herons, can cause stress on koi and other pond fish. Public domain.

Similar to above, when koi are stressed they are much more likely to lash out (just like us). In addition, stress inhibits the body’s defenses, making your koi more susceptible to illness and disease as well as bacterial infections. Stress also increases the likelihood of them relapsing into an illness that they’ve had in the past. Examples of stress include excessive handling, crowded conditions, lack of food or an otherwise inadequate diet, poor water quality, water temperatures that are too warm or cold, being housed with incompatible fish or other aquatic animal species, and competing for mates, among other things.

Common predators, such as herons, cats, and foxes, which regularly stalk the pond will also cause stress on fish if they have no shelters to hide away, so make sure you have ample predator deterrents in place.

 7) Poor Water Quality

Cloudy, unhealthy water can cause aggressive koi behavior
Poor water quality can lead to sickness, which, in turn, can lead to aggressive behavior. Public domain.

Water quality relates to many of the other items on this list. If water quality is poor with too many nutrients or pollutants like phosphorus or ammonia, or the water is even just off a bit (such as pH or water temperature that is too high or too low, increased water hardness, and so on), it’s much more likely that your koi will experience troubles. This could be illness, stress, or simply acting “off,” all of which may lead to your koi behaving aggressively toward pondmates.

8) Unsuitable Pond Companions

koi tend to be aggressive toward fancy goldfish
Studies have shown koi to be aggressive to certain other fish, in particular, fancy goldfish. Public domain.

While koi typically get along well with each other, they may not also be suited for living with some other pond fish species. For example, a very popular study known as the “Aggression of Koi Experiment” found that koi display very little actual aggression toward fellow koi aside from chasing behaviors, but tend to bite, injure, and in some cases even kill fancy goldfish. Another more scientifically backed study found similar results – when housed with some varieties of goldfish, koi are known to attack them and, for some unknown reason, seem to have a propensity for specifically targeting their eyes. As a general rule, do try to avoid keeping smaller fish species with your koi.

Koi seem to do well with golden orfes, common goldfish that are on the larger side, tenches, larger pleco varieties like the common pleco, pond sturgeons, loaches, and some catfish. Please conduct your own research before choosing to keep your koi with any other fish species so as to ensure the wellbeing of all fish in your pond. Be sure to only include fish that prefer to be within the same water quality parameters as koi, otherwise they may become unhappy and/or ill in your pond.

How to Stop Koi Fish Bullying Behaviour (Tips & Methods)

1) Separate Aggressors & Quarantine Newcomers  

There are a variety of fairly simple methods to stop or prevent koi bullying. For koi that are simply naturally aggressive, keep them in a separate pond from the other fish. You should also quarantine new arrivals for a time, perhaps via sectioning off a portion of your pond or placing them in a secure tank within your pond, so that they and your current pond residents can grow accustomed to the newcomer. There can sometimes be a bit of a pecking order with pond fish, so they’ll need time to get used to one another before being fully introduced into the pond.

2) Maintain Good Water Quality

Some other common sense solutions include never overstocking your pond, making sure there is enough (healthy) food while also spreading the food about so that everyone is able to get some, and ensuring/maintaining healthy water quality standards. You can do this by regularly cleaning excess food and plant matter out of your pond so it doesn’t cause a nutrient overload, having a quality pond filter and aeration system, performing partial water changes as needed, and conducting daily water quality tests to check temperature, pH, dissolved oxygen, ammonia, and nitrate levels at the very least.

3) Include Plants & Shelter

Incorporating a variety of native and non-toxic plants into your pond will not only help to improve water quality, but will also provide valuable shelter for your koi. This is especially useful for koi that are on the shyer side so that they can either escape from other koi that may bully, as well as allowing them a place to hide so that they themselves don’t become aggressive due to being exposed to interactions beyond the lower social threshold that their shy nature allows. Alongside plants, you can make use of artificial “koi shelters” which allow for safe and secure hiding places from other fish and predators.

4) Regularly Check Fish Health

When performing water quality tests, you should also check for any illnesses or injuries in your fish. Preventing these from occurring as well as finding and addressing them early will help avert latent violent behavior before it has a chance to transpire. As mentioned above, also be sure to either keep koi only with other koi, or with fish that they are likely to be compatible with (not fancy goldfish!).

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.

2 thoughts on “How to Stop Koi Fish Bullying (4 Steps)”

  1. I had a large koi caught in a planter, after he got out the other koi swam around him lifting him up. A large group kept swimming and nudging him. Is this normal are they trying to help him or attack him.

  2. I have watched one of the Koi bullying another. Upon closer inspection half of the fish has been chewed away by the aggressive Koi. I will remove the aggressive fish tomorrow. Then place the eaten fish in another tank. Im contemplating whether to just terminate its life. It must be in agony.


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