Koi Wounds and Lesions: Identification and Treatment

By KoiQuanta Editorial Team|Updated Mar 2026

Secondary bacterial infection of koi wounds occurs in over 70% of untreated cases within 48 hours. Any wound or lesion on a koi's body requires prompt assessment and appropriate management, not a wait-and-see approach.

KoiQuanta's wound logging tracks lesion size and healing progress with photo comparison. No competitor supports wound tracking with photo documentation the way KoiQuanta does.

TL;DR

Watch the wound closely for the first 48-72 hours for signs of infection developing.
Salt at 0.3% inhibits Saprolegnia growth.
When you photograph the wound at the same angle on day 1, day 5, and day 10, the photo comparison tells you objectively whether healing is progressing.
Keep the fish out of water for no more than 30-60 seconds at a time.
Seasonal changes require adjusted monitoring schedules; automated reminders help maintain consistency.

Identifying the Type of Wound or Lesion

The correct treatment depends on correctly identifying the type of wound or lesion. These are the main categories:

Bacterial Ulcers

Bacterial ulcers are the most serious category of koi skin lesion. They typically present as circular, open wounds with reddened edges, often with a slight central depression or crater appearance. The surrounding tissue may be reddened or hemorrhagic.

Common causes: Aeromonas hydrophila, Aeromonas salmonicida, Pseudomonas spp. These bacteria are present in virtually all koi ponds but only become pathogenic when koi immune function is compromised (by stress, poor koi pond water quality tracker, or other disease).

Treatment: Clean the wound with antiseptic solution (iodine, Betadine, or chlorhexidine) applied topically with a cotton swab. Apply an antibacterial topical (Orabase, neomycin ointment) to the wound. For severe ulcers or multiple affected fish, systemic antibiotics are appropriate if available. Improve water quality to support immune function. See the koi ulcer disease treatment guide for the full protocol.

Predator Wounds

Heron strikes produce distinctive long, narrow puncture wounds, often in the dorsal area or flanks. Cat claw marks produce parallel scratches. Raccoon attacks may cause larger tissue tears, typically from below on the ventral surface.

Distinguishing from bacterial ulcers: Predator wounds are acute (you'll often know the event occurred or find evidence), have irregular edges rather than the regular circular appearance of bacterial ulcers, and may have associated scale damage from the impact.

Treatment: Clean and disinfect the wound. Apply topical antibacterial to prevent secondary bacterial infection. Watch the wound closely for the first 48-72 hours for signs of infection developing. Add physical predator deterrents to prevent recurrence.

Spawning Injuries

Male koi chase females aggressively during spawning. The female can sustain significant physical damage: abraded scales, pressure marks, and skin damage from being pushed against hard surfaces.

Distinguishing features: Occurs specifically during spawning season (spring-early summer), affects female fish, damage pattern reflects physical pressure rather than infection, multiple areas of scale disruption without the characteristic ulcer appearance of bacterial disease.

Treatment: Similar to predator wounds: clean, disinfect, and prevent secondary infection. Separate the female from males if damage is severe. Most spawning injuries heal well with appropriate topical treatment and good water quality.

Parasitic Attachment Sites

Anchor worm (Lernaea) and fish louse (Argulus) leave characteristic attachment sites. Anchor worm creates a small wound where the parasite embeds, with the parasite visible as a white thread emerging from the skin. Fish louse creates small circular hemorrhagic spots where it attaches and feeds.

Treatment: Remove anchor worms with tweezers, rotating slightly as you pull to avoid breaking the parasite off in the wound. Treat the attachment site with antiseptic. The koi anchor worm treatment guide covers the full removal and post-removal treatment protocol. For fish louse, remove with tweezers and treat the pond with appropriate medication.

Fungal Infections

Saprolegnia fungal infections present as white to gray cotton-like growth emerging from wound sites. They're almost always secondary to an existing wound or immune compromise. The fungus colonizes damaged tissue rather than initiating infection in healthy fish.

Treatment: Treat the underlying wound and improve immune function (water quality, reduce stocking stress). Salt at 0.3% inhibits Saprolegnia growth. Malachite green topically at the infection site is effective but must be used carefully. The koi bacterial infection treatment guide also covers management of concurrent bacterial and fungal secondary infections.

Wound Assessment and Documentation

For any wound, document:

Location on the body (describe precisely or use the diagram in KoiQuanta's wound logger)
Size (measure if possible, or estimate in comparison to a known reference)
Appearance (color of edges, depth, whether there's discharge)
Photo from lateral view and close-up with scale reference

KoiQuanta's wound tracking feature creates a time-stamped record of each observation with attached photos. When you photograph the wound at the same angle on day 1, day 5, and day 10, the photo comparison tells you objectively whether healing is progressing.

Wounds that are increasing in size despite treatment require veterinary attention. Wounds that are stable or decreasing in size are responding appropriately.

Treating Koi Out of Water for Wound Care

Brief handling for wound treatment is necessary and manageable. Keep the fish out of water for no more than 30-60 seconds at a time. Work efficiently.

Wet your hands before handling. Apply topical treatment quickly. Return the fish to water and allow recovery before a second handling if necessary.

For wounds requiring more extensive topical treatment, a fish can be partially supported in a net with the treatment area just above water, allowing topical application without full removal. This is gentler than full removal and allows more time for application.

Frequently Asked Questions

How do I treat a wound on my koi?

First, identify the wound type. For bacterial ulcers, clean with a dilute antiseptic (Betadine, iodine) and apply an antibacterial topical such as neomycin ointment or orobase. For predator wounds, clean and disinfect the wound and apply topical antibacterial to prevent secondary bacterial infection. For spawning injuries, the approach is the same as predator wounds. For parasitic attachment sites, remove the parasite first, then treat the attachment wound. Maintain excellent water quality throughout treatment, as immune function is critical to wound healing. Log the wound in KoiQuanta with photos to track healing progress.

What is the difference between a bacterial ulcer and a predator wound?

Bacterial ulcers typically have regular, circular edges, a characteristic reddened rim, and often a depressed or sunken center. They develop over days, not overnight. Predator wounds have irregular edges reflecting the physical nature of the injury (puncture marks from a beak, scratches from claws), occur acutely, and may have associated scale disruption from the impact force. Predator wounds in healing can become bacterial ulcers if secondary bacterial infection isn't prevented. If you're uncertain about the origin, treat conservatively as if bacterial infection is a risk regardless of the initial cause.

Should I treat a koi wound topically or with bath treatment?

Both, in many cases. Topical treatment (directly to the wound during brief handling) is the most effective way to deliver high concentrations of antibacterial agents exactly where needed. Bath treatment (medicating the whole pond or quarantine tank) addresses the systemic bacterial challenge and prevents secondary infection through the water. For isolated wounds in otherwise healthy fish in good water quality, topical-only treatment is often sufficient. For multiple fish with wounds, rapidly spreading lesions, or water quality that's compromised, whole-pond or quarantine tank bath treatment is appropriate alongside topical care.

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Sources

Associated Koi Clubs of America (AKCA)
Koi Organisation International (KOI)
University of Florida IFAS Extension Aquaculture Program
Fish Vet Group
Water Quality Association