Koi with white spots or ulcers: ich and ulcer treatment

Fine white dots that look like grains of salt, on a koi that is flashing and rubbing, are most likely ich, a single-celled parasite. Pond treatments kill it only while it is swimming free in the water, so the cure is a series of doses spaced by water temperature: every day at 75°F and above, and several days apart in cool water. Salt, formalin and a labeled ich product all have veterinary or extension backing, used one at a time. A red open sore is usually a bacterial infection (most often Aeromonas) that took hold after stress or poor water, and it needs a water test, a swab and a vet before any antibiotic.

What the veterinary and extension sources say about ich

Each white spot is a feeding parasite under the skin. It drops off, forms a cyst on the bottom, and the cyst releases hundreds of swimmers that look for a fish. The University of Florida fact sheet (FA006) says "Only the free-swimming theronts are susceptible to chemical treatment," and for warm water the Merck Veterinary Manual says "Three to seven treatments may be required." The Southern Regional Aquaculture Center sheet (SRAC 476) says the life cycle takes "from less than 4 days (at temperatures higher than 75°F or 24°C) to more than 5 weeks (at temperatures lower than 45°F or 7°C)."

For the doses, UF gives formalin "at a concentration of 25 mg/L, which is equivalent to 1 ml of formalin per 10 gallons," with "a 'half-dose' of 12.5 mg/L" for sensitive fish, and "Vigorous aeration should be supplied during chemical treatments." For salt it gives 4 to 5 g/L (4 to 5 ppt) "in a prolonged bath for 7 to 10 days," and warns that "Plants in an aquarium or small ornamental pond may not tolerate increased salinity." SRAC lists 2 to 3 ppt of salt but reports that "Ich was observed to tolerate 3 ppt salt, while in another case, Ich withstood 5 ppt." On heat, SRAC says "Typically, Ich cannot reproduce properly at water temperatures above 85°F (30°C)," but also that in central Florida "Ich was responsible for killing fish at 92°F (33°C)," and that a swing of "15°F or more above or below the temperature at which an Ich case is detected will end the disease episode."

Malachite green is where legality comes in. Merck notes that over-the-counter pet fish remedies "often contain formalin and malachite green and are effective but, because of regulatory concerns regarding the use of malachite green, should not be dispensed by the veterinarian." On April 8, 2025 the FDA added Faunamor (methylthionine chloride, malachite green oxalate and acriflavine chloride) to its index for ornamental finfish "including tropical fish, goldfish, and koi," calling it "the only legal drug currently on the market to treat Ich." It is sold over the counter and dosed from its label.

What the sources say about ulcers

Merck's bacterial diseases page names Aeromonas salmonicida as "the causative agent of goldfish ulcer disease" and "a very important disease of koi and goldfish," ties outbreaks to "poor water quality, organic loading of the aquatic environment, handling and transport of fish," and says: "Sensitivity testing before antimicrobial use is recommended." SRAC 478 (2019) says aeromonads "are ubiquitous in the aquatic environment" and take advantage of stressed or injured fish. It warns that "Many Aeromonas isolates are resistant to one or more of the aquaculture antibiotics" and that antibiotics "should never be used to prevent disease or to treat virus or parasite caused diseases." When Goodwin and Merry (2009) ran PCR on skin swabs that hobbyists sent from 12 states, they found atypical A. salmonicida in 33 of 40 cases and put the negatives down to "high water temperatures, prior antibiotic use, poor sample quality, or misdiagnosis." Dr. Jessie Sanders, a board-certified fish veterinarian, writes at CA Fish Vet that the main stressor is "#1 = water quality," that ulcers "are diagnosed by a culture and sensitivity test," and that "Once the primary stressor has been identified and corrected, most mild ulcers will heal on their own."

What dealers and pond owners do

Ben Plonski, writing in Pond Trade Magazine (2018), quarantines new koi "two to four weeks or more," with the water "above 60 degrees, and preferably 65 to 75 degrees," adds salt "to between 0.5 to 0.6 percent, or 4 to 5 pounds per 100 gallons," and says "It is not recommended to mix salt with formalin, because this combination can be too harsh." When he uses formalin and malachite green instead, he treats "every two to three days, for a total of five treatments." Hanover Koi Farms says that for ulcers "you should heat the system to 82 degrees."

On Garden Pond Forum, kenziierae (2011) wrote "I bought 30 Koi and over half of them have died from Ich" in a 5,200 gallon pond. In that thread addy1 suggested "2-3 pounds of salt per 100 gallons" over three days, and fishin4cars said "Ich doesn't reproduce past 82 degree F." On ulcers, Lisak1 (2023) said that "ulcers are frequently a sign of a water issue," and WaterGardener warned "You never want to put antibiotics in your pond." dutchstar (2013) had an ulcer with "no ammonia, nitrite, nor nitrate," and Dave 54 told him to take swabs to a vet. Not every white mark is ich: koikeepr (2009) describes carp pox as "candle wax drippings," and Meyer Jordan (2016) read one photo as abraded scales and said "Do not salt your pond."

Where the sources disagree

The salt level for ich is the widest gap. SRAC gives 2 to 3 ppt and has seen ich survive 3 and 5 ppt, UF gives 4 to 5 ppt for 7 to 10 days, and Plonski holds 5 to 6 ppt in quarantine. addy1's 2 to 3 pounds per 100 gallons works out to 2.4 to 3.6 ppt, which is below the UF range. For heat, fishin4cars says 82°F and SRAC says 85°F, with a Florida kill at 92°F. Hanover's 82°F for ulcers has no counterpart in the veterinary pages fetched here, though Goodwin and Merry did list high water temperature as one reason swabs tested negative. The Pond Guy's ich page says that "If not treated, ich has a 100% fatality rate," which no veterinary source here states; SRAC says only that the parasite "is capable of killing large numbers of fish in a short period of time." Quarantine length ranges from SRAC's "a week or more," written for production ponds in 1998, to UF's "minimum 30-day quarantine period." The caution against combining salt and formalin comes from the trade, and the veterinary pages fetched here do not address it either way, so follow it.

What to do

1. Look closely. Ich looks like sprinkled salt, carp pox like wax, and an abrasion like missing scales. UF confirms it with a skin scrape under a microscope, which a vet or a dealer with a microscope can do.
2. Test ammonia and nitrite, and take the water temperature. Outbreaks of both diseases follow stress, and a pond still cycling is the usual source of it (see new pond syndrome).
3. Pin down the volume before dosing anything. Every amount in the table above scales with gallons, and a pond guessed at 4,000 gallons that actually holds 2,500 gets 60 percent too much.
4. Pick one ich treatment and schedule it from the temperature table. For salt, take the plants out and follow the staging and meter checks in our salt dosing guide. For formalin, run every air stone you have. For a labeled product, follow the label. Do not add formalin to salted water.
5. Do not treat an ulcer with pond antibiotics. Fix the water, then have the ulcer swabbed before any drug, since prior antibiotic use is one reason Goodwin and Merry's samples came back negative.
6. Quarantine every new koi, for 30 days per UF, at 75°F per UF's koi herpesvirus sheet, with its own nets and buckets.

When to call an aquatic vet

Call one for any open ulcer, whenever fish are dying, and for ich that has not eased by the third treatment, which is when SRAC says "Treatment effectiveness should be evaluated by a fish health professional." The World Aquatic Veterinary Medical Association keeps a Find a Fish Vet directory searchable by town, country and state, with a filter for Certified Aquatic Veterinarians. Search results show the American Association of Fish Veterinarians (fishvets.org) offers a similar tool for North America.

The PondTools salt, volume and water-change calculators

The PondTools volume calculator (also standalone) takes length, width, average depth in feet and a shape, and returns gallons, and the PondTools app gives it from a traced outline. The PondTools salt calculator (also standalone) takes that volume, your current meter reading in ppt and a target of 1, 1.5, 3 or 5 ppt, and returns pounds and 40 pound bags. The PondTools water-change calculator (also standalone) takes the volume, the current and target salt readings and the percent replaced each time, and shows how many changes bring the salt back down afterward.

Sources

• Francis-Floyd, R., Yanong, R. and Pouder, D. Ichthyophthirius multifiliis (White Spot) Infections in Fish, CIR920/FA006, UF/IFAS Extension.
• Durborow, R. M., Mitchell, A. J. and Crosby, M. D. (1998). Ich (White Spot Disease), SRAC Publication No. 476 (PDF).
• Hanson, L. A., Hemstreet, W. G. and Hawke, J. P. (2019). Motile Aeromonas Septicemia (MAS) in Fish, SRAC Publication No. 478 (PDF).
• Merck Veterinary Manual, Parasitic Diseases of Fish and Bacterial Diseases of Fish.
• Petty, D. et al. Koi Herpesvirus Disease (KHVD), VM-149/VM113, UF/IFAS Extension.
• Goodwin, A. E. and Merry, G. E. (2009). Are All Koi Ulcer Cases Associated with Infection by Atypical Aeromonas salmonicida? Journal of Aquatic Animal Health 21(2): 98 to 103.
• U.S. Food and Drug Administration (2025). FDA Announces New Drug for Treating Common Disease in Ornamental Pet Fish.
• Sanders, J., DVM. Fish Ulcers, CA Fish Vet.
• World Aquatic Veterinary Medical Association, Find a Fish Vet; American Association of Fish Veterinarians, fishvets.org.
• Plonski, B. (2018). Mixing Koi Populations, Part Two: Preventive Quarantine Protocol, Pond Trade Magazine.
• Hanover Koi Farms, Koi Treatments.
• The Pond Guy, Ich Treatment for Fish.
• Garden Pond Forum threads: My Koi have Ich (2011); Koi with ulcers on skin (2023); Ulcer and red spots on my mature koi (2013); 3 Common White Spot Diseases on Koi & Goldfish (2009); White spots on Koi (2016).