Koi Stress Treatment Guide (Lowering Stress Tips)

We are 100% reader supported. We may earn commission at no extra cost to you if you buy through a link on this page. Read our disclosure.

Share this page!

koi stress treatment guide
Koi are sensitive to changes in their environment and can easily become stressed when things begin to worsen, such as water quality. Public domain.

Koi are very sensitive fish, and can easily become stressed with sudden changes in their environment. In fact, one of the primary goals of koi keeping is trying to maintain the most stable ecosystem possible, as stability is what makes for happy koi. Sudden changes in the environment, water quality, or pond conditions can all have a negative effect on fish.

Every koi is different, however, and some will handle changes better than others, but that doesn’t mean they can’t become stressed. Eventually, even the strongest of koi will start to feel the symptoms of stress if a problem isn’t treated, and it is our job as fish keepers to try to identify the cause and improve conditions!

Some of the most common causes of stress in koi include: changes in water quality (pH/Ammonia), the arrival of predators, sickness or parasites, and even just a drastic change in environment, such as an introduction to a new pond. It could also be caused by a combination of different issues, as stress is usually a downward spiral with additional problems occurring as the fish becomes more and more affected by the stressors.

Since stress symptoms are similar in most cases, it can be difficult to pinpoint the cause unless you are able to physically confirm the problem (i.e., you have seen a heron in your garden). In most situations, it’s best to go through every possible cause, step-by-step, to ensure you’re not missing any underlying issues, and make sure koi have the most comfortable environment possible.

Common Koi Stress Symptoms 

1) Changes in behaviour

Symptoms of koi stress
Koi which turn down food, act sluggishly, or hide away from the group are likely showing signs of stress. Public domain.

One of the first things you’ll notice in a stressed koi is likely a change in behaviour, with the most common being koi becoming lethargic, hiding, or refusing to take food. All these signs could point to stress, as a stressed fish tends to break away from the group and seek shelter. You may see them swimming slowly around the deepest point of your pond, or even hidden under a fish shelter or plants.

These are places they probably feel the safest, and they’re retreating here to try to relax away from everything going on. For this same reason, you may not even see your koi during feeding time, as they’ll probably prefer eating when you’re not around when it’s quieter. These types of symptoms could point to changes in water quality, a predator problem, or just be related to a new fish in a new pond that needs time to become more confident!

2) Changes in physical appearance

koi with red sores
Rashes, sores, spots, or other appearance changes may indicate a bacterial or parasitical infection. Public domain.

Koi are known for their vibrant and vivid color patterns, and a stressed fish will gradually become more dull in appearance. As well as this, a fish which is subject to predators may have injuries, torn scales, or damaged fins. If your suspect a predator could be the cause of the stress, sometimes it’s best to carefully net your fish to the surface and give them a check. This can be done with a gentle koi sock net for maximum comfort, and may be necessary if they’re injured and require treatment.

Parasites and illness will also cause a change in both appearance and behaviour, with problems such as flukes, tapeworms, and bacterial infections being common stress inducers. These are all difficult to identify, but since most treatments for parasites and infections are very safe, they can be given at any time you suspect a problem may be occurring. Often koi keepers like to treat for both parasites and infections at the end of Autumn and the start of Spring for the best hibernation/torpor and start to the year.

Common Koi Stress Causes

1) New pond stress

chlorine stresses koi fish
New ponds will contain little beneficial bacteria and may be high in chlorine and chloramine – both possible stressors.

Fish introduced to a new environment will always be stressed to some degree while they grow used to their surroundings. Often this type of stress is nothing to be concerned about, and your koi will slowly become more confident in time. It’s normal for fish to hide for a few days in a new environment, and they may only eat food when you’ve moved away from the pond.

After a week or two, your koi should be more relaxed and start acting normally, but if they’re still hiding it would be a sign that the water quality in the new pond is imbalanced.  If you have a new pond you can reduce the amount of stress by making sure the pond is cycled correctly before adding fish.

2) Changes in water quality

Koi feel most comfortable in a stable environment, and maintaining this stability is one of the key aspects of fish keeping. A gradual change in water quality is less dangerous than a sudden change, but both can cause problems with stress. Spikes in ammonia (waste), nitrites, pH, and KH are the most common causes of stress when it comes to water quality.

These can all occur gradually, but still need to be treated the same before conditions become too severe. Rapid changes are always more dangerous to koi, as they have no time to adjust, and this causes huge amounts of stress and discomfort.

3) Sickness, injury, or parasites

Both bacterial infections and parasites will cause a fish to become stressed, with common causes being injury, fin/tail rot, flukes, lice, or tapeworms. If your fish has been injured in a predator attack or in the pond, the pond should be treated to prevent infection and harmful bacteria from growing. These treatments are usually wide-range and work for a number of bacterial problems, including fungus, eye cloud, and fin & tail rot.

Parasites are harder to determine as they’re usually much too small to see with the naked eye, but changes in behaviour when water quality is normal could point to parasites. Just like bacterial treatments most common parasites, such as gill flukes and tapeworms, can be removed with commercial wide-range treatments.

4) Predators

how to keep herons out of ponds
Herons can be a huge nuisance for fish keepers, and can cause koi both injury and stress if left unchecked. Public domain.

Koi which are constantly harassed by predators are likely to become less and less confident in their environment, and will eventually start hiding away for most of the day. Predators such as herons, hawks, cats, and raccoons will all be happy to stalk a koi pond if there is no deterrent in place to scare them off. When this happens koi will retreat to the deepest point of the pond to try to escape being eaten, and they’ll become more stressed the longer the predator stays around the pond. This becomes even more of a problem if your pond has no shelter for fish to hide, as koi will have nowhere to go to feel safe. Even after an attack, koi will need time to build confidence to come up to the surface, and if they have no “safe space” to de-stress, it can have lasting effects.

Steps to Reduce Stress in Koi

Step 1: Test Water Quality (and treat accordingly)

API POND MASTER TEST KIT Pond Water Test Kit 500-Test
  • Contains one (1) API POND MASTER TEST KIT Pond Water Test Kit 500-Test, including 6 bottles of testing solution, 3 color cards and 4 test...
  • Helps monitor water quality and prevent invisible water problems that can be harmful to fish
  • Measures ornamental pond pH balance, Ammonia content, Nitrite levels and Phosphate content

If you suspect your koi are stressed but are unsure of the cause, the first thing to do would be to test your pond’s water quality to ensure there aren’t any issues. Koi feel most comfortable in a stable environment, and a sudden change in pH or ammonia could both cause large amounts of stress on your fish. Water can be easily tested using commercial water test kits, which provide a wide range of measurements, including ammonia, pH, nitrites, nitrates, and even sometimes KH and GH (water hardness).

Even though it is good practice to perform a broad water test for all measurements, the most important and common causes for stress are changes in pH and ammonia. A rapid swing in pH can literally kill koi outright, so it’s vital that pH be kept stable and within an ideal range of 6.5-7.8. Ammonia is less likely to suddenly spike, but that doesn’t make gradually rising levels any less dangerous. Ammonia’s toxicity is also directly related to pH and water temperature, so the higher the pH and temps, the more potent and deadly ammonia becomes to fish!

If you have carried out a test of water quality and your measurements are outside the “safe” range (see here for more info), you may need to treat the pond accordingly. Ammonia is easier to adjust, and we recommend cleaning out sludge/muck, removing algae, cleaning filter media, and supplementing with beneficial bacteria. Adding more aeration and oxygen to your water is also good, as beneficial bacteria require large amounts of oxygen to break down waste.

Even if your pH is high (or low), keeping it stable is often more important than causing another sudden change. If ammonia is reduced, koi will slowly become used to the pH, and as long as you see a gradual change back to normal values over time, it should be fine to leave to re-balance. If you have a really high or low value, your pond water may benefit from some gradual adjustment through treatment.

Step 2: Treat for Infections & Parasites

API MELAFIX Freshwater Fish Bacterial Infection Remedy 64-Ounce Bottle
  • Contains one (1) API MELAFIX Freshwater Fish Bacterial Infection Remedy 64-Ounce Bottle
  • Heals bacterial infections and repairs damaged fins, ulcers and open wounds
  • Contains natural, botanical tea tree extract to quickly and rapidly help fish

Koi that are showing unusual behaviour, have dulled in color, or are showing signs of injury, may be subject to a bacterial infection or parasite. Injury can easily lead to infection, but even when it doesn’t, koi will be stressed and less confident as they try to heal their wounds. If an injury becomes infected, or the koi becomes sick, treatment is generally recommended to speed up the healing process and strengthen their immune system. This will ensure the least amount of stress during the recovery phase, and will help them get back to normal much faster.

If you notice fraying thins and an overall dulling in color, your koi may be suffering from fin & tail rot. This is a very common bacterial disease which affects both koi and goldfish, but can be treated similar to injury with wide-range anti-bacterial treatments. Luckily, fin rot is very easy to treat, and recovery is fast. Preventing fin & tail rot is always better than treating it, however, and one of the easiest ways is to ensure you’re feeding your koi a high quality fish feed with a wide nutritional profile of vitamins, proteins, and fats.

In terms of parasites, they’re a little harder to spot, but common parasites such as flukes and tapeworms usually present in sudden changes in behaviour. For example, a koi that begins rubbing on the pond liner, hiding away, or even losing weight, may have a parasite problem. Just like bacteria, parasites are easy to deal with and can be remedied with a broad range treatment. These are safe for fish and can be used whenever you feel your fish are at risk, with good times to dose being before winter torpor (“hibernation”) and at the start of spring.

Step 3: Add Predator Deterrents

Orbit 62100 Yard Enforcer Motion Activated Pest Deterrent Sprinkler
  • ULTIMATE YARD PROTECTION - Defend your yard with our powerful motion-activated sprinkler that deters animals and pests effectively.
  • SENSOR-ACTIVATED YARD SECURITY SPRINKLER - The heat and motion-activated sensor detects movement up to 40 feet away, triggering a burst of...
  • DUAL-PURPOSE SPRINKLER - Our motion-activated sprinkler not only repels animals but also allows you to add additional sprinklers to the...

Koi may be stressed due to an uninvited guest to the pond, such as a heron, who is stalking the perimeter on a regular basis. Even if you haven’t seen a predator near the pond, that doesn’t mean you don’t have one! Herons, for example, tend to visit during the very early morning and right before dusk, which means you may just be missing the action. Common predator signs include stressed koi, damaged fins, injured scales, and changes in behaviour (i.e., hiding).

Koi will naturally retreat to the deepest point of the pond when they notice a threat, and the more the threat is around their home, the longer they’ll stay there. This can slowly start to affect their personalities as well, with even the most confident koi becoming more shy over time.  Eventually, they’ll start refusing to even feed while you’re around, as they associate shadows around the water as possible predators.

The best way to solve this type of stress is to remove the predator and provide a place for fish to hide safely and relax. Herons can be stubborn, but luckily there are a variety of ways to deter them, such as netting, sprinklers, alarms, and even decoys. For more information on this check our full guide here.

Step 4: Provide Shelter & Hiding Spots

Koi Kastle 12"x24" (Aluminum and Mesh)
  • Provides a hiding place for fish
  • Makes a great retreat from intense summer sunlight.
  • 24" L x 14" W x 9" H weighs 1lb

If your koi are stressed, it may just be because they have nowhere to de-stress! A barren pond without any form of shelter provides no coverage for koi to feel safe, so adding either natural or man-made shelters could be beneficial.

Pond plants are a great way to provide natural shelter to fish, and also have the benefit of providing more oxygen, better filtration, and algae control. Floating plants, such as water lilies, and submerged plants, such as hornwort, are both good choices in terms of shelter. Water lilies create vast surface foliage and hornwort provides dense underwater forests, which also improve oxygen levels. The only downside of plants is they can take a long time to grow, so won’t be able to provide maximum benefits for several months.

If you require a shelter in the short term while plants grow in, man-made koi shelters are a great choice. These are designed to sit on the bottom of the pond floor at the deepest point, and create a very safe area to hide from predators, relax, and get away from it all. They work great alongside plants and would do well in a pond without any suitable natural shelter for fish.

Step 5: Improve Filtration and Aeration

a fountain aerates a pond
Fountains and waterfalls are a great way to provide natural water aeration and increase oxygen. Public domain.

Koi thrive in well-aerated conditions, and the more flow you can provide to the water the better! Improved aeration means more oxygen, which supports both pond fish and biological filtration. Beneficial bacteria require large amounts of oxygen to break down waste, and will be able to carry out their job much more efficiently in a high oxygen environment. Low oxygen levels can cause fish to become stressed, as they continue to search for better-aerated parts of the pond. Providing adequate oxygen to the water is an important aspect of fish keeping, and the pond should ideally be aerated across the entire system.

If you have low water flow or stagnant water, this could be the reason your koi are becoming stressed and unhappy. Adding natural aerators, such as waterfalls or fountains, will help provide oxygen and surface movement for gas exchange to take place. They will also help with the transfer of nutrients across the pond, and ensure bacteria have oxygen to grow in all areas.

If waterfalls or fountains aren’t an option, air pumps or submerged plants will both be able to provide oxygen directly to the water. Air pumps work much more quickly, but plants are good to add as a long-term solution. The more oxygen in the water, the happier your koi will be!

As well as aeration, optimizing your mechanical and biological filtration will ensure the maximum amount of ammonia and debris are removed. Ideal ammonia levels would be zero, so the closer to this you can get, the more comfortable your koi will be. Regular cleaning, adding new media, or supplementing with beneficial bacteria can all help filtration and stress levels.

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.

5 thoughts on “Koi Stress Treatment Guide (Lowering Stress Tips)”

  1. We ust moved our 3 koi to a larger pond. Water tests show everything is fine. The fish however seem to be gulping deep and at the surface. There is a fountain and a waterfall in th pond. We’re concerned but not sure what to do. The also didn’t eat their food. Help, any suggestions?

  2. We have a 3 month old pond with several Koi. About 2 weeks ago we purchased a fancy tail koi from the shop we used to build our pond. We have since noticed that this fish frequently lays on the bottom of the pond, or floats in a near-vertical position. At other times it swims with the other fish as if everything is fine. We have yet to see it eat. We spoke with the owner of the shop and found out she had a piranha in the same tank as her koi. It attacked at least one fish that ultimately died of it’s injuries. I do not know if it attacked this fish we now have. We have decided to isolate it and try to treat it, although we do not know what we would be treating it for. Can you suggest a treatment we could try? We do not have a “hospital” pond so will have to isolate it in a 27 gal plastic storage tote. FYI our other fish are all doing fine. Thanks for your help!


Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.