Koi Carp Ulcer Treatment Guide (Safe Methods)

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How to Treat Ulcers in Koi & Stop Them Coming Back (2022 Guide)

koi fish with infected ulcers
Reddening and ulceration of the scales caused by infection. Source: African Journal of Microbiology Research Vol. 6(8), pp. 1723-1729.

Even if our koi and water quality appear healthy, there is always a hidden threat lurking below the water which can strike under the right conditions. The threat we speak of is bacteria; an omnipresent micro-organism that comes in many shapes and forms, providing both beneficial properties and harmful disease. Unlike the beneficial bacteria which works to break down organic waste and make conditions better for fish, there are other bacteria strains found in ponds that can instead cause sickness, stress, and ulceration. Two of the most common are the aeromonas and pseudomonas bacteria strains, and these are the bacteria responsible for most ulcers a koi may contract.

Why is this bacteria in my pond?

Both of these bacteria strains are found naturally all ponds, but they’re usually not present in large enough quantities to cause immediate issue. As well as this, koi carp have natural slime coats and strong immune systems, which help protect them from outward infection, allowing them to withstand low levels of bacterium without issue. However, if water quality is poor, fish are malnourished, or if they become injured or wounded, problems can arise which may lead to ulcers forming.

Ulceration usually starts as a small red sore which can be hard to identity as it can form underneath the fish, inside the gills, or below the fins. Left untreated, the sore can worsen and become ulcerated as bacteria take control and spread.  Although the easiest way to identify an ulcer is visually, there are some other tell-tale signs to watch out for which can help with early identification –

Koi Ulcer Symptoms:

What Causes Ulcers in Koi and Goldfish?

gill fluke under microscope
Both gill flukes (pictured) and skin flukes are common parasites which can cause ulceration in koi and goldfish. Public domain.

As mentioned above, the most common cause of ulcers are injuries which become infected with particular strains of bacteria living in the pond. Both aeromonas and pseudomonas bacteria strains are the most common culprits, and koi will become more susceptible to infection if they’re living in poor conditions or are generally unhealthy.

Wounds can occur from parasites, such as flukes, or predator attacks, such as herons and cats. Gill and skin flukes are particularly notorious in bringing about ulcers, as unlike internal parasites (i.e. worms), they attach themselves externally and cause sores which are exposed to outer pathogens. Parasites are also very contagious, with their eggs laying dormant for several weeks before hatching, which leads to future infections in other fish. Parasites can be prevented by ensuring koi are in optimal health and water conditions are within safe parameters, but an active outbreak would still require treatment with specialised products to stop spreading.

pond bacteria that can cause fish ulcers and infection
Aeromonas hydrophila – a common bacteria found in ponds which can cause ulceration and infection. Public domain.

Predators can also cause injury to koi which can lead to ulcers, especially if you have herons in your area. Even if you don’t spot a heron in your garden, you could still be having daily visits as they like to arrive at first light and before dusk. Stressed fish, nipped fins, and damaged scales are all signs of a predator stalking the pond.

Finally, poor water conditions or general malnourishment can eventually lead to ulcers appearing as fish become sick and stressed. Poor water quality leads to more harmful bacteria growth and substance imbalance, which can have an impact on a koi’s immune system. As well as this, feeding a low quality fish food without the correct ingredients can lead to a reduction in slime coat (their first line of defense), slower healing, and less of a immune response.

Causes of Ulcers (Summarized):

  • Increase in harmful pond bacteria
  • Parasite infections (flukes)
  • Predator attacks
  • Poor water quality & conditions
  • Lack of proper nutrition (comprised slime coat)
  • Poor “winterization” routine

Are Ulcers in Koi Contagious? Should you Quarantine Sick Fish?

what causes ulcers on fish
Ulcers are not contagious, but the underlying cause could be, so quarantining can be helpful during treatment. Public domain.

Ulcers themselves are not contagious, and if other koi are healthy, the bacteria which is causing infection should not be easily contracted by other fish. However, if the cause of ulceration is parasites, such as flukes, they can be spreads between stock and cause new wounds to appear. As well as this, a sick fish with ulcers will be very stressed in general, so quarantining can be a useful practice for faster treatment and a safer recovery period – especially if water conditions are also poor.

Since koi carp are fairly large in comparison to other fish, it can be difficult to provide an effective quarantine environment and often requires the use of a specialist holding tank. Stock tanks or holding tanks are a good option for temporary quarantine as they allow you to more easily manage medication and keep an eye on recovery. If the ulceration is particularly bad, possibly internal, antibiotics may be required to treat the underlying infection. In this instance, having the sick fish in a separate tank ensures the antibiotic treatment is provided to just the sick fish, and is not able to disrupt the natural ecosystem of the main pond.

A final advantage of having a quarantine environment is you will be able to add new arrivals, such as fish and plants, to the tank before adding to your main pond. This provides a time-frame to evaluate the new stock and make sure they’re free from parasites, eggs, and harmful bacteria. A quarantine period of 3-4 weeks is good practice, as this is the average incubation period of many common parasites which can cause infection.

Treating Koi Ulcers & Slowing Infection (5-Step Guide)

Note:  Always read dosage instructions before treatment, and make sure to turn off any UV clarifiers and remove any active carbon as it will neutralize medications. When using anti-biotic medicines, quarantine is always recommended to prevent damage to the ponds eco-system.

wide spectrum antibiotics to treat koi ulcers1) Treatment with Wide-spectrum Antibiotics 

When?  At first sign of early ulceration or for advanced infections to kill external and internal bacterium.

Unless you have caught the injury or sore in the very early stages, in which case steps 2-4 may be sufficient, it’s sometimes safer to treat your koi with a wide-spectrum antibiotic medication to ensure the wound dose not ulcerate. If there are already ulcers present on the fish, or you feel infection may have spread internally, antibiotics are the only viable option available.

Kanamycin is a wide range antibiotic which is effective at killing a large spectrum of bacteria, and is also effective at stopping the spread of ulcer causing bacteria in fish. KanaPlex (containing 35% kanamycin) is a good choice for treating ulcers in koi as it can be bound to food using a binding agent, such as Seachem Focus, which ensures 100% of medication is being absorbed and the antibiotic is more effective. Although intended for aquariums, it’s easy to use with the binding agent on your regular koi feed, which can then be administered in a separate holding tank. We always recommend moving koi to a different tank when treating with antibiotics as they can drastically interfere with a ponds natural ecosystem if the medicine goes uneaten.

A single scoop of KanaPlex is mixed with 1 scoop of Focus, and this is added to a table spoon of feed (regular pellets) and a few drops of water for easier absorption. This would be carried out once a day, and then after a week of daily feeding you can evaluate the results, and if there is no improvement, you can repeat the dosing for an additional week.  Depending on the stage of infection, and the method of administration, signs of recovery should be noted within 1-2 weeks after treatment.

aquaprazi to treat koi fish parasites2) Treat for Parasites Using AquaPrazi

When? At first signs of ulcers and alongside anti-biotic treatment to reduce the spread of parasites.

If your fish are showing sign of parasites, or they have an ulcer/sore which you can’t identify the cause, treating the pond for parasites is good practice to be on the safe side. The best treatment would be a wide-range parasite killer, with our recommendation being praziquantel, as it’s particularly effective against skin and gill flukes – a major cause of ulcers. Aqua Prazi is a popular product which contains praziquantel, and can be dosed to treat a range of different parasites, both internal and external breeds. The treatment can be used on a fish in quarantine, but should also be added to the entire pond as parasites are highly contagious and may quickly spread between stock.

Luckily, Aqua Prazi is quite safe for fish when dosed correctly in good water conditions, and should have no ill effects on the ecosystem or bio-filtration. A single dose can be added to the pond over a 7 day period, and then repeated after a few weeks to combat any hatching eggs and larvae.

3) Treat fish with External Anti-bacterial MELAFIX

When?  At first sign of early ulceration/injury, or after treatment of anti-biotics during recovery phase if ulcers are advanced.

To aid in the recovery of ulcers and injury, an anti-bacterial treatment such as MELAFIX can be used to encourage the healing process and prevent further spreading. Although these kind of treatments may not be as useful in late stages of infection, they’re a good method of improving recovery during the healing stages. MELAFIX is made up from natural anti-bacterial ingredients and can be added to your quarantine tank during recovery or to the entire pond to treat multiple fish stock. Since it’s a bacterium inhibitor, it should not be used alongside KoiZyme (step 4) as it will simply destroy the harmless bacteria intended to compete with aeromonas and pseudomonas strains. The best time to use this kind of treatment would be before KoiZyme is started in the main pond, or separately in a quarantine tank where it can’t interfere with other treatments.

4) Balance Pond Bacteria Using KoiZyme

When? After main anti-biotic/anti-bacterial treatment during then recovery phase.

A rise in harmful bacteria strains, such as aeromonas and pseudomonas bacteria, can cause ulcers and infection to occur more readily in weaker fish. KoiZyme is a natural pro-biotic treatment which introduces harmless bacteria strains to the pond which out competes the growth of both aeromonas and pseudomonas bacteria and greatly reduces the chance and severity of infection. Ulcers that do appear after regular treatments with KoiZyme will often be much milder and slower to spread, with healthy fish being able to more easily fight the infection.

After a wound or injury appears you can treat the pond with KoiZyme once a day every 3 days to reduce the severity of possible ulceration. Afterwards, a maintenance dose once a week can be added to ensure that harmful bacteria levels do not increase in future.

astaxanthin can help treat and prevent bacterial infections in koi5) Provide Koi Food Containing Astaxanthin

When? At first sign of bacterial infections or constantly mixed in with normal feed for future prevention. 

Finally, an interesting topic is the use of synthetic color enhancer astaxanthin and its immune strengthening benefits against the aeromonas bacteria strains which can cause ulcers. A research study carried out by Fish and Shellfish Immunology in 2014 showed a decrease in mortality rates of carp when fed with high doses of the color enhancer food during periods of bacterial infection. With doses of 25mg, 50mg and 100mg astaxanthin added to daily feeds, both bactericidal and enzyme activity were greatly increased in fish. The study found that an enriched diet which includes astaxanthin helped modulate the immune system and growth restores against aeromonas bacteria in wild carp, with a much lowered 10-20% mortality rate compared to the control group.

Astaxanthin is a potent color enhancer popularly used in many feeds to help promote stronger color and pattern, but can be supplemented during a bout of infection to try to strengthen the immune system. It could also be mixed in with regular feed on a daily basis to provide on-going benefits against bacteria, as suggested by the research study. A good feed with high protein, vitamins, minerals, and color enhancer Astaxanthin is Iku Koi Kichi’s Growth and Color food (pictured).

How to Prevent Ulcers in Future (Pond Keeping Tips)

1) Maintain Good Water Quality

Not only will fish become weaker in poor water conditions, harmful bacteria will also thrive in your pond which can increase the risk of ulcers and infection. Maintaining good water quality through adequate filtration and general pond maintenance is a vital aspect of the koi keeping hobby. Water can be tested throughout the year with a wide-range water test kit, and any negative results can then be remedied to bring water back into safe parameters. Likewise, bottom muck and debris should be cleaned regularly and kept to a minimum, especially before koi enter their hibernation period in winter. If you have a larger pond, a quality pond vacuum is the ideal tool for cleaning, with skimmers being great for floating debris management. For further reading, check our other articles on this subject below:

2) Reduce the Risk of Parasites

If you have had your pond a long time and have only started having issues with parasites, it’s highly likely they have been brought to the pond by a foreign element. Any new fish or plants you add should be first quarantined in a separate tank for 2-4 weeks to ensure they carry no parasites or disease. To be safe, you can treat the new arrivals in the tank with a wide-range parasite killer and an external anti-bacterial treatment as described above so there is less chance of contamination with the main pond.

Parasites can also be carried to the pond by predators and pests, such as herons and ducks. If you have any visiting wildlife, you should add safeguards to stop them entering your water as parasites can be transmitted from contact or through faeces. Not only that, ducks and other birds have very potent faeces which causes all sorts of problems with water quality. In-fact, their faeces are so high in ammonia and nitrites that you can see huge swings in water quality if it’s allowed to accumulate on the pond floor. Both predators and pests can be deterred with a combination of quality pond netting and specialised deterrents.

3) Feed Nutritious Koi Food

orange koi eating nutritious fish food
Access to nutritious, quality food can help prevent ulcers, infection, and diseases from affecting your fish. Public domain.

Finally, without a strong immune system koi won’t have much chance fighting back infections even if water quality is good. The best way to ensure koi are fighting-kit for any future illness is to provide them a super high quality food with high protein, good fats, and a wide vitamin and mineral profile. Many commercial feeds you find in chain pet stores contain very low amounts of quality ingredients, and are packed with what we call “filler” content that koi don’t need in their diet. Feeding a low quality food will reduce growth, lower immune support, and can make fish more prone to sickness, parasites and disease. Your fish feed should have a quality aquatic protein source as the fish ingredient on the label, such as fish meal, krill meal or anchovy meal. Secondly, the feed should contain a decent amount of healthy fats, low ash (waste) content, and have a complex vitamin and mineral profile, in particular ascorbic acid (vitamin C). Finally, the  best feeds will also contain ingredients to enhance their immune system, such as brewers yeast, and natural parasites deterrents, such as garlic. For our recommendations on best koi feeds, check our dedicated articles below:

3 thoughts on “Koi Carp Ulcer Treatment Guide (Safe Methods)”

  1. Thank you for this VERY informative article. The only thing missing is a description — and PHOTO — of what the wound/ulcer on a healed koi should look like.



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