7 Signs of a Stressed Koi Fish and the Most Effective Remedies

Did you know that your cherished koi fish may be grappling with stress? If you’re new to koi fish, knowing when they are stressed can be hard. The best solution in such a situation is to know the telltale signs of a stressed koi fish and what to do.

A stressed koi fish may start swimming upside down while displaying behavioral changes like lethargy, loss of appetite, and gasping for air. Remedies include maintaining optimum water parameters, treating for swim bladder infections, and keeping the predators at bay.

Keep reading as we explain the signs of a stressed koi fish and what you can do to restore normalcy.

1. Swimming Upside Down

Signs of a stressed koi fish
Koi fish

Koi fish swimming upside down is one of the most stressful situations. Normally, these fish swim upright, diving toward the bottom and then rising to the top of the tank.

Stress is one of the reasons your koi fish may swim upside down.

When a fish swims upside down, it indicates that it’s undergoing some pain. Consequently, the pain deprives the fish of its natural swimming pattern.

Other potential causes of this behavior include swim bladder issues, poor water quality, and infections or parasites.

Apart from stress, koi fish swimming upside down may also indicate:

  • Swim bladder infection
  • Poor water quality
  • Parasites in the gut
  • Constipation

2. Changes in Behavior

Kois are happy and gregarious fish that rarely shy away from people. However, they can become stressed as a result of environmental issues or even territorial disputes with other koi fish.

Changes in behavior are another common sign of stress in koi fish. For example, these fish may become more aggressive, hide at the bottom of the tank, or lose interest in interactions.

If you notice any changes in your koi’s behavior, it may indicate that the fish is under stress.

A stressed koi tends to hide frequently. This is because the stress forces the fish to break away from the group in search of shelter. The fish wants a place to hide and feel safe. As a result, you’ll notice that the fish remains at the bottom of the tank most of the time.

Unfortunately, it can be difficult to pinpoint exactly what is causing this stress, as many potential culprits exist. Some of the most common causes include poor water quality and overcrowding in the tank.

3. Loss of Appetite

Koi fish are known for their voracious appetite, but a stressed koi may lose its appetite altogether. The stress hormones released in response to environmental factors or territorial disputes can cause changes in the digestive system, leading to appetite loss.

An easy way to notice when your koi fish lacks appetite is when it fails to come to the surface to eat as usual. The fish will not show interest even in flake fish food, the koi’s favorite dish.

Due to stress, the fish finds it hard to go where the food is. It might be due to fear or koi predators like raccoons and herons.

4. Erratic Swimming Patterns

Stressed koi fish often swim erratically and at unusual speeds. While these motions may seem harmless or amusing for humans, they can signify serious discomfort for koi fish.

If you notice your normally calm and gentle koi swimming erratically, it could signify stress. In most cases, this behavior occurs when the fish attempts to avoid predators in the tank.

Alternatively, erratic swimming can also indicate poor water quality or an infection.

Some erratic koi fish swimming patterns to look for include:

  • Swimming frantically in weird directions
  • Tilting sideways while swimming
  • Crashing on objects in the tank
  • Swimming at a considerably low speed.

Although these behaviors may seem harmless, they are a cause for concern. If you notice that your koi is acting erratically, it’s important to take steps to identify and address the underlying cause.

5. Loss of Coloration

The loss of the natural color is another sign of a stressed koi fish to look for.

Most Kois have orange, copper, and buttery gold hues. However, when a koi is stressed due to its tank environment or territorial disputes, it may start losing its coloration. The fish’s skin might turn pale and take on a whitish hue.

You might also notice damage to the scales due to the fish’s erratic swimming patterns that expose it to injuries. The fish will knock over or bump into objects and surfaces in the tank, causing injuries to its scales or other parts of its body.

If your koi fish is losing color or showing signs of physical damage, it may be a sign of stress. In most cases, this behavior occurs when the fish can’t find shelter in nearby rocks, plants, or other objects in the tank.

It’s worth mentioning that not all color changes in koi fish are a sign of stress. Koi fish have chromatophore cells that play a major role in colorization. These cells are also affected by seasonal changes and food intake.

An easy way to differentiate between normal color change and color change due to stress is to look for additional signs on the scales. If the fish is losing its color but the scales remain intact, it’s likely due to seasonal changes.

In contrast, if you notice that the scales are starting to chip or peel off, it may be a sign of stress. In such cases, it’s important to take steps to address the underlying cause and prevent further damage to your koi fish.

6. Gasping for Air

Koi fish needs at least 6 ppm of oxygen for healthy living. When the oxygen concentration in the water goes below this threshold, your koi fish will swim to the surface in search of more oxygen.

Insufficient oxygen in the water will stress the fish as it struggles to maintain normal function in the body. In extreme cases, the fish may even struggle to swim.

When koi fish comes to the top in search of oxygen, you’ll see it gasping for air. This is because the fish is meant to breathe through the gills, not the lungs, like humans. Thus, it will try to take as much oxygen from the surface as possible.

If you notice your koi fish gasping for air or struggling to swim, it’s a clear sign of stress. This behavior often occurs when the tank either doesn’t have a good source of oxygen or has extremely poor water quality.

Sometimes, a koi fish may experience stress due to environmental factors such as temperature changes. For instance, a sudden increase or drop in water temperature can cause respiratory problems in your koi fish.

7. Frequent Gill Movement

Gills are essential respiratory and osmoregulation organs for fish.

A fish opens its gills to let water out after extracting oxygen. However, this process doesn’t happen frequently. The fish must first open its mouth to gulp water into the gill chamber. They then close the mouth using their lips before opening the gills to let water out.

If you notice your koi fish opening its gills more frequently than normal, it’s a sign of stress. This behavior often occurs when environmental factors such as high or low pH levels or insufficient oxygen in the tank water.

It’s worth mentioning that frequent gill movement can also be a sign of bacterial gill disease. However, if this is the case, you’ll notice erosion and some damage along the edges of the gills.

For this reason, it’s important to look for other signs of stress in your koi fish to rule out bacterial gill disease.

Main Causes of Stress in Koi Fish

Everything happens for a reason, and stress in koi fish is no exception.

When you notice the above signs of a stressed koi fish, it’s your responsibility to inspect the fish, including its environment, to determine the cause and apply an appropriate remedy.

The following are the main causes of stress in koi fish you should look for:


Koi fish predator
Koi fish predator

Raccoons and herons are the main koi predators. These animals are attracted to koi fish due to their bright colors.

If these predators are around the tank or pond, they can cause serious harm to your koi fish by attacking them and stealing them from the pond.

When a koi fish notices these predators, it gets engulfed in stress and starts hiding or swimming erratically.

Poor Environmental Conditions

Keeping the fish tank water clean is important to prevent them from experiencing stress due to poor environmental conditions.

Common causes of poor environmental conditions in koi fish tanks include sudden decreases or increases in water temperature, insufficient oxygen levels, poor pH levels, and accumulation of ammonia.

High ammonia levels in your koi fish tank can be catastrophic when left unchecked. Ammonia is produced from decaying fish food and waste. If this continues, it can lead to respiratory problems in your koi fish, causing them to gasp for air at the tank’s surface or struggle to swim.

New Pond

Like humans, transferring your koi fish to a new tank or pond is associated with some level of stress. This is because fish are used to their familiar environment, and a sudden environmental change can be unsettling.

If you’re moving your koi fish from one tank or pond to another, it’s important to give them enough time to adapt to their new surroundings by introducing them slowly into the new tank or pond.

This can be done by placing a divider in the tank and gradually reducing its size until they’re used to it. After a week or two, you can remove the divider.


Spacing is vital when it comes to keeping fish, especially koi. Keeping a proper water volume ratio per fish in your tank or pond is important.

When the number of fish exceeds this ratio, it can cause overcrowding and stress for your koi fish. The stress arises from the competition for resources like:

  • Oxygen
  • Swimming space
  • Food

How to Prevent Stress in Koi Fish

As a Koi fish parent, it’s your responsibility to ensure the well-being of your fish. This means you must take steps to prevent stress from occurring in the first place.

Some of the main ways to prevent stress in your koi fish include:

Maintaining Optimum Water Parameters

Like any other fish species, koi fish depends on the aquarium’s water for food and air. However, as the fish keeps living in the water, it uses the water components like dissolved oxygen, diminishing them.

Moreover, the fish releases waste that is converted into ammonia as it stays in the tank. The breakdown of ammonia into nitrates in the tank worsens the situation by making the environment inhabitable. When this happens, your fish is bound to get stressed.

In severe scenarios, inhabitable tank conditions may be fatal.

The table below shows the optimum tank and water conditions for koi fish:

Water ConditionOptimum Level
pH6.3 to 7.1
OxygenAt least 6 ppm
Ammonia0.0913 to 0.1739
Nitrate0.065 to 0.1439
Temperature19.0 to 28.0°C (66.2 to 82.4°F)
Carbon dioxide2.9 to 6.4
Optimum water conditions for koi fish

If your koi fish’s tank condition doesn’t match the ones presented in the above table, it’s time to take the necessary steps to adjust them. Here are some of the things you can do:

  • Improve oxygen levels: If the amount of oxygen dissolved in the water is low, use aeration systems like air stones and pumps to increase it.
  • Maintain optimum temperature: If your tank water temperature is not within the optimum range for koi fish, you may need to install a temperature control system to regulate it.
  • Control accumulation of ammonia: You should perform regular water changes to reduce ammonia levels and monitor waste removal while maintaining filters in the tank. A 10 percent water change weekly is enough for smaller tanks (less than 150 gallons). Otherwise, conduct a 20 percent water change for larger tanks that exceed 150 gallons.

Keeping Predators at Bay

Kois are afraid of predators and will get stressed when they notice one. This is especially common in ponds.

Apart from stressing the fish, predators will eat the fish, exposing you to unexpected losses.

To prevent this, it’s important to install a protective fence around your pond or tank to keep predators away. This can include placing nets over the tank and installing anti-predator systems that use sounds and lights to scare them off.

Maintaining Good Nutrition

Just like any other living organism, koi fish requires the right nutrition to stay healthy and happy. A balanced diet that consists of protein, carbohydrates, vitamins, and minerals will help prevent health problems like stress.

Some ways you can maintain your koi’s good nutrition include:

  • Sticking to a balanced diet: Although Koi fish are fond of eating water plants like algae, you must ensure they have a balanced diet. A better way to achieve this is by incorporating proteinous food like earthworms in the tank.
  • Maintaining a proper feeding schedule: Ensure you feed your fish regularly without overfeeding them. A rule of thumb is to feed the fish two to three times a day. Overfeeding can cause digestive problems that lead to stress.
  • Avoiding artificial food additives: While adding artificial food additives like colors and preservatives may help the fish look prettier, they can harm their health. For this reason, you should avoid them as much as possible when feeding your koi.

Frequently Asked Questions

How Do I Reduce Stress in My Koi Fish?

You can reduce stress in your Koi fish by maintaining water temperatures between 65–77 degrees Fahrenheit (18–25 degrees Celsius), oxygen level between 6.0mg/Litre, and pH between 7.0 and 8.6. Maintaining these koi fish water parameters ensures there is nothing that can stress the fish, hence a stress-free living.

How Does Salt Help Koi?

Salt, especially Epsom salt, helps koi fish by killing bacterial and parasites on the fish’s body. Moreover, salt helps a sick or stressed koi by balancing osmotic pressure in the aquarium, helping the fish to recover.

An advantage of using salt in your koi’s aquarium is that you do not have to worry about the side effects. However, it’s worth mentioning that excessive salt can be detrimental to your koi fish. Therefore, you should only add a small amount of salt to your koi’s fish tank for safety.

How Often Should You Put Salt in Your Koi Pond?

You should space out your salt treatment in a Koi pond over a span of 3 to 4 days. While doing that, ensure you’re using 1 gram of salt per liter of water for your koi’s safety. You don’t want to overload the koi pond with salt as this can be detrimental to their well-being.

Can I Use Table Salt in Koi Pond?

You should never use table salt in Koi Pond as this will lead to the death of your fish. Table salt and any iodized salt is catastrophic to Koi fish.

It’s also worth noting that salt is toxic to koi if used in high levels and for a long period. Therefore, only use a small amount of Epsom salt.

Final Thoughts

Knowing the signs of a stressed koi fish and taking the necessary steps to prevent and treat it goes a long way in keeping your koi healthy and happy. By following the tips mentioned above, you can successfully maintain optimal water parameters, keep predators at bay, and provide your fish with good nutrition. These are the necessary steps to keep your koi fish free from stress.

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