How to Treat Koi Ulcers: Step-by-Step From Diagnosis to Healing

By KoiQuanta Editorial Team|Updated Mar 2026

Koi ulcers that are left untreated for more than 72 hours have a 3x higher mortality rate due to systemic bacterial spread. The wound itself isn't always what kills the fish, it's the secondary Aeromonas bacteria that enter the bloodstream through the open lesion.

Speed matters. But so does doing the right things in the right order. This guide covers how to treat koi ulcers from first detection through wound closure, and how to track healing progress in KoiQuanta.

TL;DR

Apply to the wound, let it contact for 30–60 seconds, then return fish to pond water.
Apply, let contact for 30 seconds, return.
Systemic antibiotic treatment is recommended for any ulcer beyond Stage 1.
Feed at the prescribed rate for the full treatment course (typically 10 days minimum).
Seasonal changes require adjusted monitoring schedules; automated reminders help maintain consistency.

Direct Answer: How to Treat a Koi Ulcer

The treatment protocol for a koi ulcer involves three components working simultaneously:

Topical antiseptic: clean and disinfect the wound directly
Systemic antibiotic: address any internal bacterial infection (via medicated food or vet-prescribed injection)
Supportive water treatment: salt at 0.3% and clean, well-oxygenated water to support the immune system

The topical treatment is the part most hobbyists skip or do inadequately. You need to physically clean the wound.

Step 1: Identify the Ulcer and Assess Severity

A koi ulcer starts as a raised red area or a spot where scales have lifted. As it progresses, you see actual tissue loss, a crater-like depression in the body wall with red, inflamed edges. Advanced ulcers may expose underlying muscle tissue.

KoiQuanta's photo-attached wound log lets you compare healing progress week over week and share with your vet at a glance. Take a photo at first detection. Take one at every treatment session. The visual record is your most useful tracking tool.

Severity assessment:

Early (Stage 1): Raised, reddened area, scales slightly lifted. Treat aggressively and outcomes are excellent.
Moderate (Stage 2): Scale loss with shallow tissue erosion, inflamed edges. Treatable at home with good technique.
Advanced (Stage 3): Deep tissue exposure, possible muscle involvement, marked inflammation. Vet consultation strongly recommended.

Step 2: Prepare for Out-of-Water Treatment

For effective topical treatment, you need to work on the fish out of water. This is one of the most common hesitations hobbyists have, but keeping the fish in the water while trying to clean a wound is both ineffective and stressful to the fish.

What you need:

A clean basin with pond water and additional salt (0.5% in the treatment container)
Net and wet towel for handling
Clove oil solution (fish-safe anaesthetic, ~40 mg/L in the holding vessel)
Cotton buds and clean gauze
Iodine, Betadine, or Tricide Neo antiseptic
Wound sealant (Propolis or Orabase if available)

Sedate the fish in a separate aerated container with clove oil solution before handling. A sedated fish is less stressed and easier to treat properly.

Step 3: Clean the Wound

Remove the fish from the anaesthetic solution. Keep it wrapped in a wet towel, never let it dry out. Work quickly but carefully.

Cleaning process:

Use cotton buds to gently remove any necrotic (dead) tissue at the ulcer edges. It will look whitish or grey-green, distinct from healthy pink-red tissue.
Clean the wound with a cotton swab soaked in dilute iodine solution or Betadine. Apply gently to the wound crater.
If scales around the ulcer are loose but not fully detached, leave them in place, they provide some protection. Remove only fully dead scales that will fall off with minimal handling.

Step 4: Apply Topical Treatment

After cleaning, apply your chosen antiseptic directly:

Tricide Neo: A commonly used wound preparation that combines chloramine-T and other antimicrobials. Apply to the wound, let it contact for 30–60 seconds, then return fish to pond water.

Betadine/Povidone Iodine: Direct application to the wound. This is one of the safest topical antiseptics for koi. Apply, let contact for 30 seconds, return.

Propolis spray: A natural antiseptic with wound-healing properties. Apply as a surface sealant after the primary antiseptic. Provides some physical barrier.

What antiseptic is safe for koi ulcers? Tricide Neo, Betadine (povidone iodine), and Propolis are the most widely used. Avoid alcohol-based antiseptics, they damage healing tissue. Hydrogen peroxide should not be used on koi wounds.

Step 5: Systemic Treatment

Topical care treats the surface. But Aeromonas, the primary cause of most koi ulcers, can penetrate the bloodstream through an active wound. Systemic antibiotic treatment is recommended for any ulcer beyond Stage 1.

Medicated food: The most practical option for hobbyists. Medikoi Medicated food or similar products containing oxytetracycline are widely available. Feed at the prescribed rate for the full treatment course (typically 10 days minimum).

Injected antibiotics: For severe or rapidly progressing ulcers, an aquatic vet can administer amoxicillin, enrofloxacin, or other antibiotics by injection. This is the highest-efficacy option for advanced cases.

Step 6: Optimise Water Quality

A koi with an active ulcer has a compromised immune system. Water quality becomes even more critical than usual.

Salt at 0.3% reduces osmotic stress and has mild antiseptic properties
Ammonia at zero, any ammonia exposure during recovery sharply worsens outcomes
High dissolved oxygen, increase aeration during treatment
Temperature, avoid temperature swings; stable at 20–22°C is ideal for bacterial immune response

AquaticVet software is clinic-focused and expensive. KoiQuanta gives hobbyists a structured ulcer treatment protocol at $39/mo, including the ulcer treatment program with daily monitoring prompts and a connected bacterial infection treatment tracker.

Step 7: Monitor Healing Progress

Check the wound every 2–3 days during active treatment. Photo log every check. Signs of improvement:

Wound edges starting to turn pink rather than inflamed red
Granulation tissue forming (small pinkish-white tissue growth at wound edges)
No increase in wound size
Fish behavioural improvement (feeding, activity)

Signs that treatment isn't working:

Wound enlarging
Edges darkening or showing necrosis
Fish going off food, sitting on the bottom
Other fish developing lesions

If you're not seeing improvement within 5–7 days, consult an aquatic vet. This is when a second opinion can save the fish.

What does a koi ulcer look like?

A koi ulcer starts as a raised, reddened area with slightly lifted scales. As it progresses it becomes a depressed, crater-like wound with inflamed edges, visible tissue erosion, and often a whitish or yellowish discharge from the wound site. Advanced ulcers may expose underlying muscle tissue. Early-stage ulcers are sometimes confused with scale injuries or bruising, the key distinguishing sign is inflammation and progressive tissue loss. KoiQuanta's wound log lets you photograph each inspection so you can track progression objectively.

Should I take my koi out of the pond to treat an ulcer?

Yes, for effective topical treatment, you need to work on the fish out of water. In-water treatment attempts are rarely thorough enough to properly clean and treat the wound. Use a clove oil anaesthetic solution to sedate the fish before handling, work quickly with the fish wrapped in a wet towel, and return it to pond water as soon as you're done. The short out-of-water period during a proper treatment is far less stressful than leaving a wound untreated.

What antiseptic is safe to use on koi ulcers?

The safest and most effective options are Tricide Neo (a commercial koi wound treatment), povidone iodine (Betadine), and Propolis spray. All three can be applied directly to the wound without causing additional tissue damage. Avoid alcohol-based antiseptics, they damage healing tissue. Do not use hydrogen peroxide on koi wounds. After applying your primary antiseptic, a Propolis-based wound sealant provides an additional protective layer between treatment sessions.

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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

Track Every Healing Step

The ulcer treatment program in KoiQuanta guides you through every step, logs each treatment session, and generates a photo-linked healing timeline. Connect to the bacterial infection treatment tracker for systemic antibiotic monitoring.

Start your free KoiQuanta trial and treat your first ulcer with confidence.