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Treatments overview

By the koi.wiki editorial team · Updated

Key facts: Koi keepers use a limited set of chemical treatments — salt, potassium permanganate, formalin, praziquantel, antibiotics and hydrogen peroxide — each suited to particular parasites, bacteria or stressors, with dosing and safety constraints described by fisheries extension publications.[1][2]
Treatment of koi health problems typically relies on a small number of chemicals and drug classes that have been studied in broader fish-health and aquaculture literature, including salt, potassium permanganate, formalin, praziquantel, antibiotics and hydrogen peroxide.[3] Sources emphasize that treatment should follow accurate identification of the problem, since dosing, legality and effectiveness vary by chemical, by target organism and by system type (pond, tank or recirculating system).[8][1]Information here is drawn from university extension and aquaculture research publications rather than koi-specific veterinary guidance; for diagnosis of a specific case, sources recommend involving a fish health specialist or veterinarian before treating.[8][3] See also the page on diagnosis and on parasites of koi.

Salt in koi ponds

Salt (sodium chloride, NaCl) is approved under an FDA GRAS declaration for use on food fish as an osmoregulatory enhancer; it causes fish to release mucus from skin and gills, which can remove or kill some external parasites at higher concentrations, while lower concentrations reduce osmotic stress during handling and hauling.[1] Most tropical fish can tolerate 1–3 g/L without harming a biological filter, according to one extension publication on recirculating systems.[4]For Ichthyophthirius multifiliis (white spot/Ich), a prolonged bath of 4–5 g/L (4–5 ppt) salt for 7 to 10 days at warmer water temperatures is described as effective in smaller systems, since the free-swimming theront stage is intolerant of increased salinity of 3–5 ppt; most freshwater fish tolerate 5 ppt for several weeks and many can live permanently at 3 ppt, though species-specific tolerance should be checked.[2] A separate publication notes 2–3 parts per thousand of salt used indefinitely as one of the best preventives for Ich.[7] By contrast, increased salt concentration does not appear effective against Argulus (fish lice) in freshwater.[11] For anchor worm (Lernaea), salt in the aquarium at 1–2 tablespoons may help prevent secondary infection, though manual removal and other treatments are more directly effective against the parasite itself.[12]According to the US importer Kodama Koi Farm, salt should generally be used only when needed rather than continuously, with quarantine or temporary tanks being a common use case to increase a fish's natural disease resistance; the source also advises against combining salt with formalin, stating the combination can be harmful to koi, and recommends non-iodized salt rather than table or iodized salt.[10] This dealer source also notes that pond plants vary widely in salt tolerance and that plants are sometimes removed from ponds before salt is added.[10]In prophylactic bath trials prior to shipping, salt at 3 ppt for 11.5 hours did not significantly change fish appearance or behavior compared to untreated controls, unlike formalin, potassium permanganate, oxytetracycline or nitrofurazone, which did show improvement.[13] Extension sources also caution that sudden changes in salt concentration can decrease biofilter efficiency in recirculating systems.[14]

Potassium permanganate

Potassium permanganate (KMnO4) is a strong oxidizing agent that is exempted from FDA registration for use in food fish ponds and is categorized by the FDA as a low-priority, "regulatory action deferred" chemical rather than formally approved.[1][6] It has been used experimentally against external parasites and bacteria including Costia, Trichodina, Ambiphrya, Chilodonella and gill flukes, typically at 2 ppm; concentrations above 2 ppm can become toxic, especially in water with low organic content, and it can kill algae, risking subsequent oxygen depletion.[1]Dosing varies by target: for general external infections and columnaris or water molds on damaged tissue, a prolonged bath of 2–4 hours at 2 mg/L or a short bath of 10–30 minutes at 10 mg/L is described, with 1 mg/L sometimes used in aquariums with little organic matter.[5] For anchor worm (Lernaea), a 30-minute bath at 25 mg/L is reported to kill larvae, though adults may survive.[15] For fish lice (Argulus), 10 mg/L for 30 minutes, or 1.3 mg/L applied twice over three days, has shown effectiveness against juvenile and adult stages in freshwater, but it is not recommended in marine systems.[11] For Ich specifically, one source advises avoiding potassium permanganate because repeated treatments needed in a short period make it a more dangerous choice, cautioning against use more than once a week due to risk of damage to skin, gills and eyes.[2]Potassium permanganate is also described as able to inhibit nitrifying bacteria in biological filters to some degree, alongside formalin, copper sulfate and certain antibiotics.[14][4] It is additionally used as a prophylactic bath before shipping ornamental fish, typically for about 4 hours, where it was rated among the more effective chemicals tested for improving post-transport appearance and behavior.[13]

Formalin

Formalin is commonly used to treat Ich in tank systems, though it is described as not ideal for ponds.[2] FDA-approved formalin products are labeled for external parasites on all species and life stages, typically applied at 25 mg/L (equivalent to about 1 mL of formalin per 10 gallons of system water), with a half-dose of 12.5 mg/L available for formalin-sensitive species.[2] Vigorous aeration is recommended during treatment, particularly where gills are already damaged, and sick fish showing stress or attempting to jump should have the chemical flushed immediately with clean, oxygenated water.[2]As a pond treatment, formalin is used at 15 to 25 ppm, equivalent to roughly 4.5 to 7.5 gallons of formalin per acre-foot of water.[7] Formalin has also been shown to inhibit nitrifying bacteria in biological filters to some degree.[14] One source dealing with koi care specifically warns against combining salt and formalin, stating the mixture could be harmful to koi.[10] Dactylogyrus and Gyrodactylus flukes are also listed among parasites for which formalin baths are used as one of several chemical options.[16]

Praziquantel

Praziquantel is an anthelmintic medication used against parasitic worm infections in a range of animals, including fish, and works partly by affecting the function of a worm's sucker.[17] In veterinary use it has been applied against Diplozoon paradoxum and other trematode infections in many fish species, and in 2025 the European Medicines Agency's veterinary committee recommended authorization of a praziquantel-based product for monogenean gill infestations in sea bream.[17] For Dactylogyrus (gill flukes) affecting cyprinid fish, praziquantel is listed as one of several chemical treatment options alongside salt baths, formalin, organophosphates and potassium permanganate.[16]

Antibiotics

Extension guidance states that affected fish should not be treated with any antibiotic until a sample from several fish showing typical disease signs has been cultured and analyzed, since prior antibiotic exposure can produce poor culture results.[8] Legal availability of antibiotics for ornamental fish is broader than for food fish because the FDA has used regulatory discretion to keep some aquarium products available, though the agency is working to expand indexed drug options.[8]Bath (immersion) antibiotic treatments require much higher drug amounts than oral treatment or injection to achieve an effective dose in the fish, and excess antibiotic in water can promote resistant bacteria; 70–100% water changes are recommended at the end of each daily bath treatment and before redosing.[8] Bath treatments are not recommended in recirculating systems or where treated water would contact a biological filter, since antibiotics can kill or inhibit nitrifying bacteria; studies cited found bath antibiotic treatment reduced biofilter function by 44–100%.[8][4] As a result, treated fish are generally isolated or taken off-line, with full water changes afterward.[4]Among specific drug classes, penicillins are most effective against gram-positive bacteria and are not FDA-approved for food fish, making them a poor first choice for most fish bacterial infections.[8] Oxytetracycline is broad-spectrum and works well mixed into food, but calcium and magnesium bind and inactivate it, meaning harder water requires higher bath doses, and some species have not absorbed expected levels from bath exposure.[8] Nitrofurans are used in baths mainly for superficial infections and can be inactivated by light, while potentiated sulfa drugs such as Romet remain effective, unlike some older, more commonly resisted sulfas.[8] Oxytetracycline and tetracyclines were also effective in prophylactic shipping-bath trials, alongside potassium permanganate, formalin and nitrofurazone, whereas acriflavine neutral performed worse than no treatment at all.[13] For columnaris disease, tetracyclines and quinolines are used, including medicated food containing oxytetracycline for internal infections, although resistance is noted as emerging.[18]

Hydrogen peroxide

Hydrogen peroxide (H2O2) is used as an immersion bath treatment in aquaculture against external parasites, bacteria and fungi, and the FDA has approved one hydrogen-peroxide-based aquaculture product, 35% PEROX-AID.[9] Over-the-counter human hydrogen peroxide is typically sold at 3% active ingredient, far lower than the approved aquaculture product's 35% strength, and only the 35% product is legal and has established dosing formulas; use of non-approved hydrogen peroxide products is described as illegal.[9]Factors such as light, heat, high pH and various compounds accelerate hydrogen peroxide's breakdown in water, and improper dosing can cause toxicity or ineffectiveness; species vary in sensitivity, with some experiencing reduced growth rates or gill damage at high doses, and specific mortality thresholds have been documented in channel catfish and bluegill.[9] Columnaris treatment literature also lists hydrogen peroxide, alongside potassium permanganate and copper sulfate, as usable externally on adult fish and fry, while cautioning about toxicity at high concentrations.[18] More research is still needed on hydrogen peroxide's safety and efficacy, including effects on biofiltration, in recirculating systems.[4]

Other chemicals and treatment practices

Other chemicals mentioned in aquaculture fish-health literature include acetic acid (vinegar), used as a short dip of 1,000–2,000 ppm for one to ten minutes as a parasiticide, and copper sulfate, used as a pond or bath treatment for Ich calculated according to water alkalinity.[1][7] For Argulus (fish lice), organophosphates such as trichlorfon and insect growth regulators such as diflubenzuron have been used historically, though some products are no longer manufactured or are restricted-use.[11]Before any treatment, a small-scale bioassay — applying the proposed chemical dose to a sample of fish from the same water — is recommended to check for toxicity, run for 24 to 96 hours depending on whether the chemical's breakdown time is known.[1] Facilities are also advised to maintain stocks of common treatment chemicals such as salt, potassium permanganate and formalin, and to have a designated veterinarian familiar with the species who can prescribe treatments.[3] See also the pages on quarantine and diagnosis for how fish are assessed before treatment is chosen.

Common questions

Is potassium permanganate safe to use regularly on koi?
Potassium permanganate is not recommended for repeated frequent use; for Ich specifically, one source advises against using it more than once a week because of the risk of damage to skin, gills and eyes.[2] It is a strong oxidizer that can be toxic above about 2 ppm, especially in low-organic water.[1]
Can salt and formalin be used together in a koi pond?
According to the US importer Kodama Koi Farm, salt and formalin should never be combined, as the mixture could be harmful to koi.[10]
Do antibiotic baths harm pond filtration?
Yes; bath (immersion) antibiotic treatment is not recommended in recirculating systems because it can kill or inhibit nitrifying bacteria in the biological filter, and studies found such treatments reduced biofilter function by 44–100%.[8][4]
What concentration of hydrogen peroxide is approved for fish treatment?
The only FDA-approved aquaculture hydrogen peroxide product, 35% PEROX-AID, is formulated at 35% active ingredient, far stronger than the roughly 3% strength of over-the-counter human hydrogen peroxide, and dosing formulas apply specifically to that approved product.[9]
Why should fish be diagnosed before starting antibiotic treatment?
Extension guidance states that samples from several affected fish should be cultured before antibiotics are given, because fish already treated with antibiotics often produce poor, less useful culture results for identifying the correct drug.[8]

References

  1. ^ SRAC Publication No. 410: Calculating Treatments for Ponds and Tanks. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/410%20Calculating%20Treatments%…. Retrieved November 2026.
  2. ^ UF/IFAS EDIS FA006: Ichthyophthirius multifiliis (White Spot) Infections in Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA006. Retrieved November 2026.
  3. ^ SRAC Publication No. 472: Investigating a Fish Die-Off and Submitting a Sample. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/472%20Investigating%20a%20Fish%…. Retrieved November 2026.
  4. ^ UF/IFAS EDIS FA100: Fish Health Management Considerations in Recirculating Aquaculture Systems—Part 2: Pathogens. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA100. Retrieved November 2026.
  5. ^ UF/IFAS EDIS FA033: Monogenean Parasites of Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA033. Retrieved November 2026.
  6. ^ Potassium permanganate (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Potassium_permanganate. Retrieved November 2026.
  7. ^ SRAC Publication No. 476: Ich (White Spot Disease). Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/476%20Ich%20(White%20Spot%20Dis…. Retrieved November 2026.
  8. ^ UF/IFAS EDIS FA084: Use of Antibiotics in Ornamental Fish Aquaculture. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA084. Retrieved November 2026.
  9. ^ UF/IFAS EDIS FA157: Use of Hydrogen Peroxide in Finfish Aquaculture. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA157. Retrieved November 2026.
  10. ^ Benefits of Salt in Koi Pond & Tips for Safely Using It!. Kodama Koi Farm (koi dealer). kodamakoifarm.com/benefits-salt-koi-pond-why-should-use-it/. Retrieved November 2026.
  11. ^ UF/IFAS EDIS FA184: Argulus (Fish Louse) Infections in Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA184. Retrieved November 2026.
  12. ^ Lernaea (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Lernaea. Retrieved November 2026.
  13. ^ UF/IFAS EDIS FA120: Preparation of Ornamental Fish for Shipping. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA120. Retrieved November 2026.
  14. ^ SRAC Publication No. 452: Recirculating Aquaculture Tank Production Systems: Management. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/452%20Recirculating%20Aquacultu…. Retrieved November 2026.
  15. ^ UF/IFAS EDIS FA185: Lernaea (Anchorworm) Infestations in Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA185. Retrieved November 2026.
  16. ^ Dactylogyrus (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Dactylogyrus. Retrieved November 2026.
  17. ^ Praziquantel (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Praziquantel. Retrieved November 2026.
  18. ^ Columnaris (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Columnaris. Retrieved November 2026.