Koi & Goldfish Disease Info

Koi and goldfish disease is a broad topic that is the basis of many books. Instead of handling it comprehensively, we will hit on the topics that are important to the average Koi owner. As an example, while you will not find microscopic photos of Koi Parasites with detailed explanations of their life cycles here, you will find descriptions of how infected Koi will behave and recommendations on what you should do to treat them and/or what you can do to avoid the problem in the future.

The most important concept to understand with fish health is that many parasites and diseases are present in every Koi pond, but they only trigger a disease episode when the fish are under some sort of stress, resulting from poor water conditions, rough handling, bad nutrition or harassment by a predator. Therefore, when a disease episode is triggered, it is absolutely critical to determine the cause, and fix it so that after you treat the disease it does not have the opportunity to reoccur.

Koi & Goldfish health is also highly dependent on good nutrition, water quality and environmental conditions. Therefore, we have put together a page covering each of these issues. Visit them at the following links:

Bacterial Disease

Ulcer Disease (synonyms: Red Sore, Red Mouth, Hole-In-Body Disease, Dropsy, Pine-Cone Disease, Pop Eye)

This condition is usually caused by Aeromonas hydrophilla, but other species of Aeromonas as well as Pseudomonas and a few less common bacterial genera can cause the disease. Generally, an infection is triggered by injury or environmental stressors. Common disease signs are ulcers, red sores, red mouth, or (in more advanced cases) the fish can swell up internally, giving the appearance of a pine-cone, with its scales and/or eyes popping out.

Treatment of this disease became much more difficult when the government banned medicated food for animals without a Veterinarian Feed Directive (VFD).  What that means, is that in order to obtain medicated food to treat a disease, you must first bring in a veterinarian to assess the situation, and basically write a prescription for the fish, a VFD.  Unfortunately, in many cases, the cost of doing this exceeds the value of the fish, so pond owners will face very tough decisions.  In the past, Medicated Koi Feeds were generally available that proved very effective at treating this condition.  Now most of them are no longer available, even with a VFD.   

So for now, the most effective treatment, by far, is unrealistic for the vast majority of pond owners.  What you are left with is prevention and supportive care.

Koi in particular are very susceptible to bacterial infection, probably because they have been inbred so much to develop the unique color patterns.  The best thing you can do to avoid bacterial disease is to not handle the fish when the water is colder than 70F.  Reason being is that the immune system of Koi is almost nonexistent when the water temperatures are below this.  They do not establish a vigorous immune response until the water temperatures top 75F.

If a bacterial disease does develop, probably the best treatment is supportive care with Pond Salt and Melafix.   The issue is that Pond Salt kills aquatic plants above a certain concentration (different for different plants).  So apply Pond Salt in ponds with valued plants at your own risk.  People say that you can use up to 1-1/2 pounds per 100 gallons if you have plants, but different people will have different results.  If you do not have plants, or are not concerned about their well being, a beneficial treatment for Koi is 3 pounds of Pond Salt per 100 gallons of pond water.  DO NOT USE REGULAR SALT as some ingredients can be toxic to fish.  Maintain salt level monitoring with a Koi Medic Salinity Meter until the condition has improved, and then do partial water changes, to reduce the salt concentration.  Please be aware that salt does not evaporate from water, so once you put it in, its levels will not drop unless you remove and add water.

Finally, this disease commonly occurs as a result of infection at the site of a sore caused by parasites. Consider doing a Shot-gun treatment to kill all parasites. See the Parasitic Disease section below for details.


Columnaris Disease (Synonyms: Saddleback Disease, Fin Rot, Cotton Wool Disease)

This disease is caused by the bacteria Flexibacter columnaris and related species.  It usually appears as erosive lesions on the head, back (hence saddleback disease) and fins (especially the tail fin). It commonly occurs as a result of rough handling in warm (above 65 Fahrenheit) water.

Treatment options (or lack there of) are the same as those described above for Ulcer Disease.

Fungal Disease

Saprolegniasis (Synonym: Fungus)

This disorder is usually caused by the fungus Saprolegnia sp. or one of a few similar types. It appears as small patches of cotton wool, usually at the site of an injury, and may take on a green appearance with the accumulation of algal cells.

As with Columnaris, this disease usually results from rough handling or some other adverse condition for the fish. Before treatments are initiated, steps should be taken to correct the conditions that caused the disease.

Although it will not directly treat the fungus, Pond Salt is one of the best treatments to help the fish heal themselves. See the treatment information above for Ulcer Disease for proper use of Pond Salt.

An all natural treatment for Saprolegnia is Pimafix. This product can be effectively used in conjunction with Pond Salt and Melafix.

Parasitic Disease

Protozoan Parasites

A protozoan parasite is a unicellular organism that, for at least one stage of its life cycle, parasitizes fish. The infamous Ich is the only freshwater protozoan fish parasite that is visible with the naked eye. Without the aid of a microscope, all others can only be distinguished by symptoms.

Ichthyophthiriosis (Synonym: Ich, White Spot Disease)

This disease is caused by the infamous protozoan Ichthyophthirius multifiliis, AKA Ich. It is the only freshwater protozoan fish parasite that can be seen with the naked eye, appearing as very small white spots covering the body and fins. There are a number of treatment options available for this parasite, but perhaps none more effective than the Formalin and Malachite Green combination found in Ich-X (click on this link for treatment details).

Also, 3 pounds of Pond Salt per 100 gallons of pond water (0.3%) can be used to treat Ich, but it probably will not eliminate it. Maintain salt level for a couple weeks monitoring with a Koi Medic Salinity Meter and then do a partial water change to reduce the salt concentration.

Probably the best treatment option is applying Ich-X followed by Pond Salt after the final water change, maintaining it at 0.3 to 0.5% for a few weeks. This will theoretically kill the parasite and all its life stages. However, Ich is a real bugger and follow-up treatments are commonly required if conditions that initially caused the outbreak are not corrected.

When treating for Ich, always watch the fish carefully and be prepared to flush the pond quickly if the fish appear stressed. Also, be sure to aerate heavily with any of these treatments, remove as much organic debris as is practicable and treat the make-up water with a Water Conditioner. Finally, Ich-X should not harm plants or biofilters if applied as directed, but salt treatments could. To limit the effect of this on the biofilter, add 1 pound of salt per 100 gallons, wait 12 to 24 hours and add another 1 pound per 100 gallons, wait 12-24 hours and add the final 1 pound per 100 gallons. Most (not all) plants will tolerate 1-1/2 pounds of salt per 100 gallons (0.15%), but 3 pounds per 100 gallons may harm them.

Other Common Freshwater Protozoan Fish Parasites

Costia (Itchthyobodo necatrix), Chilodonella and Trichodina are all motile protozoans that commonly occur on Goldfish and Koi. All of them are too small to be seen with the human eye, but they should be suspected if the fish appear depressed, show respiratory distress, have clamped fins, and/or have excessive mucus production, which causes a white film over the surface of the body. The same treatments outlined for Ich can be used against these parasites, but the salt treatment alone may not be entirely effective for some of them, especially Costia.

Flukes (Synonyms: Dactylogyridiasis, Gill Flukes, Gyrodactylidiasis, Skin Flukes)

Flukes are microscopic Monogenetic Trematodes that commonly occur in ponds. Gyrodactylids usually occur on the skin and fins and Dactylogyrids usually occur on the gills, but either could be found anywhere on the external surfaces of the fish. Mortality is usually associated with outbreaks under crowded conditions, but fish with small amounts of these parasites can lack any symptoms but still suffer damage. The Flukes attach themselves to the fish with large hooks and feed on their blood or epithelial cells. Strains of bacteria that cause Ulcer Disease have been isolated from the mouths of Flukes, so their feeding behavior may not directly kill the fish but it can, and often does, trigger bacterial infections.

Perhaps the safest and most effective treatment for Flukes is Praziquantel. In fact, there is not a Koi or Goldfish that is up for sale at Keystone Hatcheries that has not been through a Praziquantel treatment. It is probably the safest treatment that you will ever find for ponds. Simply remove your activated carbon (if you have any) and dose the pond – this treatment will not affect your biofilter or plants!

Anchor Worms

Actually a crustacean, not a worm, Lernaea (AKA Anchor Worms) is a copepod that can be found on some freshwater pond fish. They appear as small (typically less than an inch) cream-colored threads hanging off the side of the fish. The major threat from these parasites is the wound that they leave, which is very susceptible to Ulcer Disease. Some people choose to remove these parasites with tweezers while the fish is anesthetized. This is a good idea, but probably not for less-equipped typical pond owners.

Probably the most common treatment for this parasite is the active ingredient Diflubenzuron. However, products with that active ingredient are no longer available because of government restrictions. Currently, we are recommending treatment with the product called CyroPro. This product is very effective, but it takes awhile to see results because it kills the juvenile stages of the parasite, not the adult stages. After 3 weeks of treatment as directed, the fish should be cleared of the disease.

Fish Lice

These pesky parasites called Argulus have prominent eyes and a sucking disc and flat disc-shaped body ¼” to ½” in diameter, making them easily visible with the naked eye. Treatments outlined above for Anchor Worms will work for these buggers too.

Shot Gun Treatment for Parasites

If you suspect parasites, but you do not know which one(s), or if you want to do a prophylactic treatment when adding new fish, you can use the following treatment regime:

1. Apply one ounce of Ich-X per 120 gallons of pond water.

2. Wait 8 to 24 hours (not less nor more), do a 33% water change, treat makeup water with a Water Conditioner (if necessary) and repeat #1.

3. Wait another 8 to 24 hours (not less nor more) repeat #2.

4. 24 hours after your final Ich-X treatment, apply Praziquantel and CyroPro as directed.

5. Seven AND 14 days after initial CyroPro treatment, re-apply as directed.

Note: Aerate vigorously throughout treatment period, especially during the 3 Ich-X treatments. Monitor water quality throughout treatment period and be prepared to perform a major water change at any time if problems arise. Please note that any time you do a major water change, you should be careful not to change the water temperature by more than 5F in a day. Greater temperature changes can seriously stress your fish.

Also, if you do not have plants, you can enhance the treatment with the use of Pond Salt. To do so, when you add the Praziquantel and first CyroPro treatment, also add 1 pound of salt per 100 gallons of water. Then 24 and 48 hours later, add an additional pound of salt per 100 gallons of water. This will give you a total of 3 pounds of Pond Salt per 100 gallons of water. The spacing out of salt additions prevents shocking (killing) the beneficial bacteria in your pond.

Viral Disease

There are two viruses that are a major health threat to Koi in the United States. Both are contagious and can kill healthy fish. Since there are no real treatments for Koi Viral Diseases, your entire focus should be on avoiding ever contracting the virus in the first place. By being sure that your Koi are coming from a source that is free of Koi Herpes Virus and Spring Viremia of Carp you can avoid a world of problems. All of our Koi are either hatched and raised at Keystone Hatcheries or one other wholesale-only Koi farm that does not import. Neither location has ever had a disease episode with these viruses, and both facilities have active annual virus testing programs in place. On top of that, we do not bring anyone else’s Koi onto our farm for any reason, such as over-wintering, spawning or resale. We feel that these safeguards provide a level of protection from the viruses. Ask your Koi supplier what measures they take.

Spring Viremia of Carp (Synonyms: SVC, Infectious Dropsy, Viral Hemorrhagic Septicemia)

This disease has caused problems in Europe and the Middle East for years and years, but has only recently been diagnosed in North America. First, following a positive test result for the virus, a major Koi Farm in the United States was quarantined by the federal government and all their fish were destroyed. Then, following a large Carp die-off in a northern Wisconsin lake, SVC was diagnosed in the wild in North America for the first time. The virus is very difficult to diagnose, and many businesses that sell Koi avoid testing to prevent being shut down by the government, so it is very possible that this virus has been around for quite awhile.

As the name suggests, SVC typically occurs in Koi as they come out of winter in water temperatures between 52 and 65F. Symptoms can be confusing because Ulcer Disease commonly occurs at the same time. However, infected fish commonly appear dark, have pop-eye and a swollen abdomen. They are often lethargic and can display a loss of equilibrium with occasional erratic swimming.

If your pond water temperature is in the 52-65F range and your fish are exhibiting the symptoms listed above, there are two things that you can do: first, treat the fish for Ulcer Disease in case that is the problem. Second, consider installing a Pond Heater to get the water temperature above 70F, at which temperature the Koi are able to mount an immune response and fight the virus.

Koi Herpes Virus (Synonyms: KHV, Koi Pox, CyHV-3)

This virus is genetically very similar to the more benign Carp Pox, but it is by far more deadly. It has emerged in recent years, causing mortality rates of up to 90%. There is a great deal that is still unknown about this virus, such as exactly how it spreads and if surviving fish carry it, but it is known that the virus is highly contagious and typically causes disease outbreaks, especially in young Koi, when water temperatures are between 72 and 81F.

Signs of KHV are often non-specific and mortality can begin before symptoms are apparent. However, signs can include white blotches on the gills, bleeding gills, sunken eyes and blotchy patches on the skin.

If your pond water temperature is between 72 and 81F and the fish are exhibiting the signs described above, you have two choices, and we are not endorsing either option. The first is to destroy your fish and bleach anything that was exposed to the fish or pond water at 200 ppm Chlorine for at least an hour. The second option is to raise the water temperature to 86F, at which temperature the fish develop a natural resistance to the virus. However, there is a potential problem with option #2. That is, it may be the case that fish that survived exposure to the virus become carriers, which could easily re-infect any new fish.