Koi swimming in a pond

We have recently been posting about the different types of bacteria and parasites that are commonly found in your koi fish.

Here is a combined list of those common illnesses and “diseases” we have posted about, highlighting how you can identify and treat them.

NOTE: Whenever you use chemicals or medications to help combat a particular affliction ALWAYS, ALWAYS use the manufacturer’s recommended dosage. Failure to do so can result in the death of the koi in treatment as well as its pond or tank mates.


Click on the name to jump to the topic



Fish Lice


Anchor Worms




Carp Pox

Lesion caused by Aeromonas


Type: Bacteria

AKAAeromonas hydrophila, Ulcer Disease, Hole In the Side disease

How to Diagnose

  • Flashing
  • Lethargy
  • Loss of appetite
  • A small red or white pimple forms on the skin
  • A lesion with scales missing

How to Treat

  • Quarantine
  • Raise water temperatures to 80-82ºF
  • Feed Medicated food
  • Clean the wound with Hydrogen peroxide
  • Add topical antibacterial or inject baytril
  • Bathe in tricide-neo
  • Raise salinity

Chilodonella under a microscopeChilodonella

Type: Protozoan parasite

AKALernaea elegans

How to Diagnose

  • Need a microscope for proper diagnosis
  • Ear- or heart-shaped with cilia (small hairlike structures) on the ventral side
  • Fins clamped
  • Flashing
  • Rubbing
  • Gasping at the surface
  • Floating on their sides until disturbed
  • More time near airstones or waterfalls (increased oxygen areas)
  • Increased mucous production
  • Skin looks cloudy
  • Appetite loss
  • Weight loss

How to Treat

  • Standard parasite protocols are effective
  • Malachite Green and Formalin
  • Potassium permanganate bath
  • Salt dip (2.5%)
  • Salt bath (0.5%)
  • Extra aeration
  • Smaller doses of food

Fish lice under low magnficationFish Lice

Type: Crustacean ectoparasite

AKAArgulus foliaceus

How to Diagnose

  • Visible to the naked eye
  • Look for pencil-lead sized, algae-colored, horseshoe-crab shaped creatures
  • Rubbing
  • Flashing
  • lethargy
  • Appetite loss
  • Red lesions (where the parasite has attached)

How to Treat

  • Dab Potassium permanganate on the lice directly
  • Carefully remove lice with tweezers
  • Use topical antibacterial ointment on open lesions
  • Medicated food
  • Follow up treatments may be necessary due to egg life cycle and adult-stage hardiness

Ich under a microscopeIch

Type: Protozoan parasite

AKAIchthyophthirius multifilis

How to Diagnose

  • Need a microscope for proper diagnosis
  • U- or horseshoe-shaped nucleus in a round body
  • Flashing (rubbing against the sides or bottom of the pond bottom),
  • Lethargy
  • Appetite loss
  • Small white spots like grains of salt

How to Treat

  • Difficult to treat while on the host koi
  • Salt solution of 0.3%-0.5% at no more than 80ºF
  • Raise salinity slowly to acclimatize fish
  • Monitor water temps for salting periods
    • 50ºF 14-21 days
    • 60ºF from 10-12 days,
    • 65-70ºF from 7-8 days
    • 75-80ºF 2-5 days
  • Try malachite green and formalin if salt is ineffective
  • Clean media where possible
  • Gradual 30-40% water changes

anchorworm in a koiAnchor Worms

Type: Crustacean parasite

AKA: Lernaea elegans

How to Diagnose

  • Visible to the naked eye
  • Look like greenish white threads or pieces of floss
  • Flashing
  • Red, inflamed spots (where the parasite is embedded)
  • Ulcerations on the skin
  • Difficulty breathing
  • Lethargy
  • Loss of appetite

How to Treat

  • Standard parasite protocols are most effective
  • Potassium permanganate—either as a dip in a quarantine tank or entire pond (5-10ppm)
  • Dimilin
  • Formalin (exercise caution if it has open wounds, though)
  • Salt dip (to lessen secondary infection)
  • Anchor worms can be removed with tweezers
  • Grab them as close to the entry point as possible
  • Make sure that the entire parasite is pulled
  • Disinfect the wound afterwards (to minimize infection)
  • Only keep the koi out of the water for a few seconds at a stretch (and especially if it is already stressed)
  • Removal may cause more stress and trauma than leaving it in, so treating them chemically can be a better alternative

Columnaris under a microscopeColumnaris

Type: Bacteria

AKA: Cottonmouth, Fin Rot, Tail Rot, Saddleback disease, Flavobacterium columnare

How to Diagnose

  • The fins and tail look ragged
  • Sores or lesions will develop on the body, eventually becoming ulcers
  • A lesion around the dorsal fin is common, banding around the fish
  • Koi develops a white milky sheen in patches over the body
  • Discolored mucous around the mouth, eyes and on the head and dorsal regions.
  • Gills will become a discolored, brown color
  • Breathing shallowly and rapidly
  • Lethargy
  • Appetite and weight loss
  • Lot of time near the surface
  • Resting for long periods on the pond/tank bottom

How to Treat

  • Quarantine immediately
  • Lower the tank temperature to reduce virulence
  • Start with 0.5% solution salt bath
  • 30-50% water change to reduce risk of spreading rapidly through the water column
  • Clean the pond floor media to remove as much waste as possible
  • Potassium Permanganate bath is the most common treatment
  • Treating external infections with hydrogen peroxide
  • Use Nitrofuran-based antibiotics (like Nitrofurazone or Furazolidone)
  • Feed koi antibiotic medicated foods
  • Finish the course to limit chance for future resistance
  • Clean all tanks thoroughly

costia under a micrscopeCostia

Type: Flagellate protozoan parasite

AKAIchthyobodo necatrix

How to Diagnose

  • Need a microscope for proper diagnosis
  • Look for fast-moving, kidney-shaped body with two flagella
  • Loss of appetite
  • Lethargy (long periods of not moving)
  • Fin clamping
  • Labored (or “heavy”) breathing
  • Rubbing
  • Red lesions with white slime
  • Increased slime production (which appears blue-grey in color)
  • Mucous coming out of the gills

How to Treat

  • Start with single Potassium Permanganate treatment
    • should not be used if there is gill damage
    • turn off UV light (where applicable)
  • Salt treatment (either in a separate bath or as a long-term pond treatment)
  • 3% Hydrogen peroxide on the open lesions to slow its growth
  • Single dose of anti-parasite treatments (like malachite green and formalin)
  • Raise the water temperature to the low 90ºF range

Dropsy with pine coning of the scalesDropsy

Type: Multifaceted causes (viral, parasitic bacterial, congenital), but damage mostly due to bacterial infection

AKA: Pinecone, Bloater disease, Pop-eye disease

How to Diagnose

  • Eyes bulging out
  • Distended (bloated) stomach
  • Raised scales (so it looks like a pine cone
  • Gasping
  • Unbalanced swimming (as the swim bladder is affected by the excess fluid, too)
  • Appetite loss
  • Once these physical symptoms are noticeable, it is usually too late

How to Treat

  • Act quickly
  • Isolate affected koi
  • Raise water temps
  • Increase oxygenation
  • Adopt antibacterial regimen
  • Antibiotic regimen may include tetracycline, naladixic acid, penicillin or baytril
  • Use antibiotics (either injected or in feed)

Carp PoxCarp Pox on a koi fish

Type: Virus

AKA: Cyprinid Herpesvirus 1

How to Diagnose:

  • White, grey, blue or pinkish spots that look like molten wax
  • Look for spots on head, shoulders and fins (sometimes they will cover the whole body)

How to Treat

  • As a virus, and without an effective antiviral option, prevention of an outbreak is the best treatment
  • Reduce potential stressors:
    • lower your fish load
    • optimal water parameters
    • high dissolved oxygen levels
    • be vigilant of ammonia
    • monitor pH closely
  • Periodic Chloramine T dosing (reduces opportunistic pathogen instance)

3 responses

  1. Rita Gimeabreak :

    However, you CAN use metronizadole for fish. It’s the same thing, but for fish. Follow the directions on the bottle that is for fish dosage. It’s used for bacterial infections.

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