Guide to Koi Pond Fish Fungus Identification & Treatment 2023

When your koi are ill or otherwise not acting like themselves, your first thought is likely that they’re simply under the weather or have some sort of virus or bacterial infection. However, there’s a chance that it could be a fungal infection, and while these are typically easier to identify and therefore treat, they should be taken just as seriously as they can be extremely dangerous and lead to death if left untreated.

What Types of Fungus Affect Pond Fish? 

1) Saprolegnia (Very Common)

Most fungi that impact koi and other pond fish belong to the family Saprolegniaceae, which is a family of freshwater molds. Members of this family are quite hardy, able to grow in a wide range of pH parameters, temperatures, and even in high salinity. However, sudden and lasting changes in your water quality can enable these, and other fungi, to grow much more easily.

Within this family, the fungi that typically affect koi are species that belong to the following genera: Saprolegnia, Achyla, and Aphanomyces. Among these, the most common culprits are the Saprolegnia species, also known as the cotton wool disease due to their cotton-like appearance when growing on fish, and can make an appearance anywhere on the scales, fins, gills and even as mouth  fungus.

2) Dermocystidium (Uncommon)

Dermocystidium is another genus of parasitic protists that are typically grouped in with fungi due to their similar appearance and functions, though they, like Ichthyophonus below, are not in fact fungi. Most literature that you find on Dermocystidium infections still classifies them as fungal, so we’ve included this genus in the article as well. Dermocystidium koi is the species that is responsible for infecting koi in particular.

While D. koi isn’t known to be fatal and is rare, it does cause painful skin lesions that can, and likely will, disrupt your koi’s quality of life and so should be treated immediately. It also weakens your koi and can enable secondary infections to move in, which may be fatal. Transmission and infection occurs when the zoospores of D. Koi, which are free-floating freshwater organisms, encounter the gills or open wound of a fish that they can then latch onto. Therefore, it must be present in the water or in a fish placed into the pond in order to spread to your koi.

3) Ichthyophonus (Very Rare)

Finally, another rare type belongs to the genus Ichthyophonus, which does not actually contain molds or fungi but is often grouped with molds due to similar causation, symptoms, and appearance in fish. In fact, Ichthyophonus are unicellular eukaryotic parasites that have fairly recently been found to be related to both fungi and animals, making them members of the unique kingdom known as Protista (protists), which are neither plants, animals, nor fungi but a group all on their own.

Ichthyophonus hoferi (not to be confused with the “ich” parasite) in particular lead to the disease ichthyophoniasis, which is found in both captive and wild fish of many species, including koi. It’s considered a marine disease, so higher salt concentrations are typically needed for this disease to emerge and spread, though there are some reports of it occurring in freshwater fish. Once infected, even after symptoms have disappeared, the fish will be a carrier for life and can infect other fish, though transmission in koi seems to only occur via ingestion of carrier fish or those that are actively infected.

What Causes Fungal Infections in Koi?

Fungal infections are caused by a variety of common-sense factors. Essentially, if your pond and/or fish aren’t healthy, fungal infections may arise. Poor water quality with too high or low of pH or temperature for your fish, too much ammonia or other pollutants, or too much decaying matter that adds too many nutrients to your pond are a hotbed for many illnesses, whether they’re fungal, bacterial, or viral.

If your fish are already sick, have open wounds, or are stressed, they’re systems are very much compromised and this can enable fungi to easily establish themselves on and even in your fish. As a general rule of thumb, do not overstock your pond, feed your koi a balanced diet, and regularly test and maintain water quality.

Common Symptoms of Fungal Infections:

• Mold-like white growths on fish, particularly around the mouth, gills, or any wounds
• Lesions or ulcers on skin
• Lethargy
• Immune system suppression (and therefore reduced appetite or loss of appetite)
• Loss of protective mucous coating
• Gasping at the surface for air
• Dull coloration
• Weight loss and emaciation
• Erratic or unusual swimming patterns
• Enlarged internal organs with lesions (this can, of course, only be discovered via necropsy after death.)
• Not interacting with other fish
• Presence of other illnesses, such as a bacterial infection, that are most often secondary after the host fish has been weakened by fungus

How to Treat Koi Fish Fungal Infections (Treatment Comparison)

Raising the salt level by .1 to .3% (or 1 to 3 grams of salt per liter of water) can help aid your fish in osmo-regulatory balance by disrupting the life cycle of the fungus. This is most commonly used to treat saprolegniasis, or cotton wool disease, which, as mentioned above, is the most common fungal infection in koi. Since the cotton molds can withstand increased salt levels, you may need to increase salinity by as much as 1 to 2.5% in order to actually kill off the Saprolegnia.

Only add more salt if you do not notice any improvement after approximately a week, as adding too much salt right away can harm your koi. If you raise the salt level to 2% or higher, don’t keep the fish in this solution for longer than a half hour or so. Salt baths are more likely to work on parasitic infections, such as the protist ones mentioned above that are often mistaken for fungus, but are not generally incredibly effective against fungus. If after a salt bath the symptoms still persist, then your fish has a true fungal infection and you should seek out alternate treatment.

Malachite green and formalin are often mixed together to treat fish fungal infections. If you get a cream, this can be applied directly to the affected area(s) of the fish. It also comes in a liquid variety, which is used to create a bath for the fish. Typically, a concentration of .05 ppm (5 mL per 10 gallons of water) is enough to take care of Saprolegnia as well as the protist parasites discussed above.

Each day, perform a 25% water change and add more of the malachite green/formalin mixture. Once you can no longer see fungus growing or persisting on fish, you should be able to return fish to the pond. Make absolutely certain that you purchase the zinc-free malachite green variety, as zinc can be quite toxic to fish. Do not exceed 1 ppm of malachite green in the water, as this can be toxic to fish. If the bath contains 1 ppm of malachite green, only soak the fish for up to an hour. The perk of malachite green and formalin is that they typically quite effectively take care of fungi, but they can persist in fish for months after treatment and are toxic if not dosed correctly.

Potassium permanganate is available as a crystal or powder and is mixed into a bucket of water for dilution before adding to the quarantine tank. It acts by essentially chemically burning off fungus, parasites, and algae while having minimal negative impacts on fish. Through this process, potassium permanganate will also use up oxygen, so it’s essential that you properly aerate the water to prevent fish and water hypoxia.

To treat your koi, soak them in a bath for 4 hours with a potassium permanganate concentration of 2 milligrams per liter of water (2 ppm). At this time frame and dosage, you can repeat as needed. If you dose more than this, your fish may become stressed or even develop burns. Some sources and product labels recommend using 4 ppm of product, but many hobbyists report that this has caused as many as 50% of their fish dying, so a half dose is more advisable to be safe.

A fairly non-aggressive treatment for fungal and bacterial infections as well as parasites like the protists covered in the first section of this article, acriflavine can be dosed as a bath at approximately 10 mL per 150 gallons of water, or 200 mg per 10 gallons of water. At this 5 ppm dose, you can leave the koi in the bath for up to 5 days if needed.

If fungal infections are severe, you can increase the dosage to 10 ppm (37.8 mg per gallon of water) but do not let your koi remain in the bath for longer than 2 hours at the most, as they will likely start to show signs of stress beyond this point. On the downside, acriflavine will deplete beneficial bacterial and nitrogen, which is why it should not be used for extended periods of time.

Chloramine-T, though very effective against fungal infections, should never be used as a high-dose dip, as it can cause extensive damage to fish skin and gills. As with copper sulfate, chloramine-t is quite toxic in soft, acidic water, and therefore the dosage depends on your water quality parameters. Please consult this chart to get an idea of how you should dose your quarantine tank. Make sure to continually aerate the tank during treatment, as the chloramine can deplete oxygen levels.

References:

(1) Investigations in associated protozoa-bacterial infections of cyprinids from a fish farm situated on the Jijia river in N-E of Romania – Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Crucian-carp-Cyst-in-ventral-and-caudal-areas_fig2_265297756