How to Treat Fin Rot in Koi (Fish Safe Treatments)

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 carp with fin rot
Fin rot can be caused by both bacteria and fungus and is usually a sign of poor water quality conditions. Public domain.

Fin and tail rot can be a very distressing condition, but luckily it’s something that is easy to remedy if caught early and treated correctly!

Fin rot often presents itself as a symptom of an underlying problem, and it is very rare for the disease to occur on its own. Even though fin rot can also affect wild carp, it is much more common in ornamental koi due to their weaker immune systems and their enclosed living environment.

Fin rot is usually caused by a bacterial infection that results in the rotting of fin tissue, but some cases can also be caused by fungal infections. Fin rot caused by bacteria will present as an uneven pattern that looks like the fins are slowly rotting away from the edges. If the cause is due to fungus, the rotting appears more evenly and is likely to produce a white color along the edges of the fins. Both types of fin rot are dangerous, but both can be solved with commercial treatments and pond care if caught in the early stages.

If you suspect your koi may have fin rot, you should not leave it to chance – guide them to the surface with a soft net and take a closer look to be sure!

Common symptoms of fin and tail rot in koi include:

  • Fraying or rotting edges of fins
  • Discolouration of fins (black/brown/white)
  • Inflammation around the base of the fins
  • Missing fins or parts falling off
  • Changes in behaviour (shyness, hiding, rubbing)
  • Changes in color (dulling/redness)

Koi Fin Rot Medicine & Prevention

Note – Before treating water with medicines, turn off any UV clarifiers and stop activated carbon filtration. These will neutralize treatments!

Step 1: Stop the rot with treatment

API POND PIMAFIX Antifungal Pond Fish Infection Remedy 64-Ounce Bottle
  • Contains one (1) API POND PIMAFIX Antifungal Pond Fish Infection Remedy 64-Ounce Bottle
  • Rapidly and safely treats fungal infections on body and fins of fish
  • Contains all-natural extract from West Indian Bay Trees to help with fungus and cottony growth

The first thing you need to do when you notice signs of fin rot is to treat the condition with a wide range medication. These types of medicines will be able to treat a range of problems, including bacterial and fungal infections. They will also help with the healing of open wounds, and prevent any further infection by strengthening the koi’s immune system. Even though you will also need to investigate the cause of the fin rot, treating it first is always good practice so it can’t spread deeper within the fish. Once treatment is applied, you can start looking at what may have caused the problem in the first place and how you can fix it.

As a starting treatment, we recommend a broad spectrum medicine that should be enough to help most koi recover from fin rot. These are safe for fish and will not cause issues with water balance or clarity. For more advanced cases of fin rot where the disease is spreading close to the base, you can also try antibiotic food treatments. We would first advise a regular treatment before antibiotics, as these can cause problems with water quality and may contribute to bacteria that become more resistant to medicine. It may be necessary, however, if your koi is not responding to normal treatments, such as in cases where the disease has spread internally.

Since koi are much larger than other hobby fish, it’s not always an option to quarantine them away from the rest of the group. However, fin rot is very unlikely to spread if you manage to treat the underlying cause, and quarantining just the sick fish will not always help the others if the problem is located in the main pond system (poor water quality). The best approach would be to first treat the sick fish showing signs of rot, and then you try to locate and treat the underlying cause so it doesn’t come back in the future.

Step 2: Be wary of possible parasites

Aqua Meds Aqua Prazi 50 grams
  • Koi Pond Treatment for Fluke parasites

Fin rot may have started because of a parasitical infection, or their condition could worsen if they manage to get parasites during a period of fin rot where they’re weaker and more vulnerable. Since parasites are very difficult to diagnose, many koi keepers decide to treat their fish for parasites regularly throughout the year as a precautionary measure. The most common times to do this would be before winter when koi enter torpor, and during early spring when koi start to become more active again. They can also be applied to the pond when you notice changes in behaviour, such as koi hiding, not eating, or rubbing against the pond liner. These are all potential signs of parasites, and if the behaviour doesn’t improve, you can treat your koi just in case!

Even if you think your fin rot was caused by water quality, or a predator injury, that doesn’t mean parasites won’t soon become a problem. Koi are more likely to get parasites during periods of weakness, and they’ll certainly be more at risk if they happen to have a nasty case of fin rot. This is also why koi are treated for parasites during winter and spring, because during colder weather their metabolisms and immune systems are slowed, which increases the chances of infection.

Just like anti-bacterial treatments, parasites can often be handled with a wide spectrum solution. We recommend Aqua Meds Aqua Prazi, which covers most common parasites, including gill flukes, tapeworms, and anchor worms. The treatment is very safe for fish and will not cause issues with clarity or filtration, nor does it require any water changes in advance. After the koi have been treated for fin rot with an anti-bacterial medicine, treatment for parasites is recommended to ensure the best possible recovery.

Step 3: Take steps to prevent fin rot in future

Dainichi Koi Food - All-Season (5.5 Lbs), Medium (5.5 mm) Floating Pellet
  • Encapsulated with our unique vitamin and mineral coating after the cooking phase
  • Made Fresh to Order in the USA
  • Active digestive enzymes and no fillers promote digestion rates up to 90%

Fin rot rarely occurs by itself and is almost always a symptom of an underlying problem with your koi or pond environment. To prevent fin rot from coming back in the future you will need to try to identify the potential cause, and then try to implement safeguards.

Since water quality usually plays a major part in the cause of problems in ponds, testing your water quality should be the first step in solving the puzzle. This can be done alongside fin rot treatment, as most medicines won’t interfere with water tests (although antibiotics might). When performing a water test you’ll need to pay close attention to measurements for ammonia, nitrites, and pH – all of which can cause trouble when they’re out of balance. For more information on testing water quality and what your ideal values should be, refer to our full guide here. The article also includes ways to improve the condition of water if certain values are too low or high.

Alongside water quality, you should also check your pond’s filtration system. If your water tests are showing ammonia and nitrites are higher than normal, it likely points to an issue with your filter box and biological filtration process. Making sure your filter box is large enough for your fish stock, and ensuring you have sufficient beneficial bacteria in your pond are important to control ammonia levels. You can also try optimizing and cleaning filter media if it’s clogged or under-performing, which can become a regular problem in older filter media.

As well as this, koi should be as happy as possible in their environment! Stress in fish can lead to all sorts of problems, including fin rot, so any potential stress inducers should be kept to a minimum. You should also make sure you’re feeding your koi the best quality food possible to ensure they maintain a strong immune system and plenty of energy to fight back disease and infection.

Articles and further reading to help you prevent fin rot and other diseases in koi:

Preventing problems is always better than trying to fix them, so taking steps to improve the condition of your pond and fish is very important. A well-fed koi in a high-quality water environment with no predators or other stress inducers is at much less risk of fin rot and other problems. Remember, a happy koi is a healthy koi!

What Causes Fin and Tail Rot in Koi?

what causes fin rot
Fin rot is often a result of poor water quality, lack of nutrition, stress, or injury from predators. Public domain.

The disease itself can be due to both bacterial infection (Pseudomonas fluorescens) or fungal infection, and it is commonly a symptom of another problem and does not simply occur without reason. For example, by far the most common cause of fin rot in koi is poor water quality and living conditions; which can include high ammonia, nitrites, or rapid changes in pH. Koi are very sensitive fish, and will quickly become stressed if their environment is heavily out of balance. This stress will negatively affect their immune system, cause them to weaken, and eventually it will lead to other problems – such as fin rot.

As well as poor water quality, fin rot can occur due to an injury that has become infected, or it could be caused by a parasite problem which has been left untreated. Predators, such as herons, are the most frequent cause of fin damage, but the koi could also damage themselves while swimming or during feeding.

Parasites are more tricky to spot, but if your koi have changed in behaviour recently and dulled in color, this could be the cause of the disease. Even though you cannot see most parasites, behavioural changes such as not eating, hiding, flashing, or rubbing against the pond liner are all tell-tale parasite signs.

A final cause could be due to lack of proper nutrition and an overall poor diet. Feeding koi with low-quality food will result in them weakening over time, and this leaves them open to infection and disease. One of the most important parts of fish keeping is ensuring koi get the correct nutrition for optimal growth, color, and immune support, and a high-quality feed is at the heart of this process.

Can my koi recover without treatment?

Treating and preventing koi fin rot tail rot
Unlike wild carp, koi have nowhere to escape from poor quality conditions or problems, so they’re unlikely to recover without help. Public domain.

Sometimes koi will be able to recover, and sometimes they won’t – do you really want to take that chance? The problem with fin rot is the root cause is often something in the koi’s environment, so unless something improves in the environment the disease will only worsen over time. In wild carp, who have stronger immune systems anyway, they can move away from poor quality conditions, bad feeding grounds, or predators, as they’re not enclosed in a small area of water.

Domesticated fish don’t have this luxury, and if the fin rot is due to poor water quality, predators, or parasites, they have nowhere else to go! Helping your koi fight the problem with proper medication, and removing all potential causes of the disease is always the best way forward.

Since the disease is often an indicator of a problem elsewhere in your pond, you should treat it as an early warning that could be a potential problem brewing in your pond that needs investigation.

Will the fins grow back after medication?

Yes! After a koi is treated their fins should eventually grow back in healthy condition so long as no other problems remain. Feeding the koi a highly nutritious diet and maintaining excellent water quality is important during the healing phase for the best possible recovery.

In most cases of fin rot, the disease does not have time to progress to the base of the fin, so no bone tissue is involved. However, in very severe cases the bacteria or fungus could infect the bones of the fish, and this would require further medical care from a professional keeper or veterinarian. Once the disease has progressed this far, removing infected flesh and bone is often required for recovery, and this is not something we recommend a new fish keeper carry out by themselves.

So long as the fin rot is caught early and has not spread to the base of the fin or body, treatment with anti-bacterial/fungal medicine should be enough to clear the problem and allow fins to grow back healthy.

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.

7 thoughts on “How to Treat Fin Rot in Koi (Fish Safe Treatments)”

  1. This was a really well written article. I like the way you briefly explained about the cause, symptoms and care. I enjoyed reading your article. I like the way you wrote about disease in detail. Thanks for sharing the article with us. My goldfish suffered from fin rot disease month ago and I read your article. and when i corrected my water parameter the disease was gone. did 30% water change every week until he recovered. turned out, the worst thing you do to your pet fish is fail to give the water parameter that you fish requires. I learned my lesson.
    Thanks for your help.

  2. Help! I have quarantined one fosh. Tail fin is almost gone and completely covered in what looks like a white cloud. He has been treated with Melafix and the other one Para?? One for fungal and one for bacterial. My son seems to think what we see in the tail may be a scab of sorts from it healing because it doesnt come off.but we have been trating now for days and doing changes. Any ideas?

  3. Hi yes very informative I have keeped fish for over forty years and with spawning I get about five or six fish a year carp koi crosses my pond is about 15 ft X 12 ft X 6 ft deep this year I have tried collecting the spawn and rearing the fry two months after spawning I have about 250 carp koi crosses they are about 2-3 inches in size now they are in a pond I have used as a filter pond full of shrimp and bloodworm the fry have eaten every thing in there a few of the fry are showing signs of fin rot both ponds are the same water which I test regularly and it is fine have you any idea what is chasing the fin rot before I go ahead and treat it with chemicals


Leave a Comment

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