Aeromonas Bacterial Infection in Koi: Treatment Guide

By KoiQuanta Editorial Team|Updated Mar 2026

Aeromonas hydrophila is present in virtually every koi pond. Under normal conditions with healthy fish and stable koi pond water quality tracker, it's opportunistic - it waits. Give it an opportunity - a fish stressed by transport, an ammonia spike, a wound from a predator attack, water temperature in the disease-sweet-spot of 10–20°C - and Aeromonas moves fast.

I've lost more fish to Aeromonas than any other disease. Not because it's untreatable - it's very treatable when caught early - but because of how quickly it progresses from "something seems slightly off" to a 3cm open ulcer exposing muscle tissue.

TL;DR

Give it an opportunity - a fish stressed by transport, an ammonia spike, a wound from a predator attack, water temperature in the disease-sweet-spot of 10–20°C - and Aeromonas moves fast.
Not because it's untreatable - it's very treatable when caught early - but because of how quickly it progresses from "something seems slightly off" to a 3cm open ulcer exposing muscle tissue.
Net the fish and keep it in a wet net or damp towel - don't let it thrash 2.
With the ulcer area out of water, debride loose or dead tissue gently with sterile gauze 3.
Apply topical antiseptic to the wound: iodine-based (povidone-iodine), chlorhexidine, or proprietary koi wound sealants 4.
Avoid in hard water above 250 mg/L CaCO3 (calcium bonds tetracycline and reduces efficacy).
14–21 days for established ulcers or hemorrhagic cases.

What Aeromonas Looks Like

The clinical presentation varies by the route of infection and the immune status of the fish:

Early skin/ulcer form:

Small reddened areas on the body, often starting at fin bases
Scale lifting over a small area (1–3 scales)
A slightly sunken area of skin that looks "dull" compared to surrounding scales
The fish may be slightly off its food or showing mildly reduced activity

Developed ulcer:

Frank open wound with reddish, inflamed edges
White or cream-colored dead tissue at the ulcer center
May have a "punched-out" appearance with clean edges, or ragged edges with spreading hemorrhage
Secondary fungal infection (Saprolegnia) can appear as white cotton-like growth around the edges

Hemorrhagic form:

Redness spreading across large body areas, particularly at fin bases
Bloody streaking in fins
Sometimes associated with water quality crashes (ammonia spikes trigger this response)
Fast-progressing - can move from minor redness to systemic hemorrhage in 48–72 hours

Dropsy (systemic/internal form):

Raised scales that stand out from the body giving a "pinecone" appearance
Belly swelling (fluid accumulation in the body cavity)
This is the most serious presentation - survival rate is under 30% even with aggressive treatment
Indicates Aeromonas has moved from a local skin infection to a systemic internal infection

Is It Aeromonas or Something Else?

Aeromonas presents similarly to Pseudomonas and Columnaris. Key differentiators:

Pseudomonas tends to cause fin erosion and more diffuse body redness, less commonly classic punched-out ulcers
Columnaris (Flavobacterium) causes saddle-shaped lesions on the back, especially near the dorsal fin, and progresses faster (can kill within 48 hours in warm water)
KHV can cause similar necrotic lesions but typically includes gill necrosis and the characteristic KHV mucus overproduction

For serious or atypical presentations, culture and sensitivity testing from an aquatic veterinarian will definitively identify the pathogen and guide antibiotic selection. Antibiotic resistance in Aeromonas is increasing - particularly to older compounds like ampicillin and erythromycin.

Treatment Protocol

Step 1: Isolation

Any fish showing Aeromonas signs goes into quarantine immediately if it isn't already there. Aeromonas is shed into the water and infects other fish through skin abrasions - a fish with an open ulcer is actively seeding the pond.

Step 2: Wound Care (If Ulcer Present)

For fish with open ulcers:

Net the fish and keep it in a wet net or damp towel - don't let it thrash
With the ulcer area out of water, debride loose or dead tissue gently with sterile gauze
Apply topical antiseptic to the wound: iodine-based (povidone-iodine), chlorhexidine, or proprietary koi wound sealants
Some practitioners apply an antibiotic wound gel (tetracycline ointment) to the wound surface before returning the fish to the tank

This surface treatment doesn't replace systemic antibiotic therapy - it addresses the local wound environment and helps prevent secondary Saprolegnia infection.

Step 3: Systemic Antibiotic Treatment

For any ulcer larger than 1cm, or any hemorrhagic or dropsy presentation, systemic antibiotics are needed. The surface treatment alone won't clear an established Aeromonas infection.

Oxytetracycline (Terramycin Aquatic): Bath treatment at 10–20 mg/L for 10 days minimum. Avoid in hard water above 250 mg/L CaCO3 (calcium bonds tetracycline and reduces efficacy). Water changes before treatment reduce organic interference.

Enrofloxacin (Baytril): Requires veterinary prescription. Better penetration into infected tissue. Available as injectable (more reliable for serious cases) or bath treatment. Increasingly used for Aeromonas that shows tetracycline resistance on culture.

Florfenicol: Prescription required. Effective broad-spectrum antibiotic used in aquaculture. Good tissue penetration.

Minimum course duration: 10–14 days for mild cases. 14–21 days for established ulcers or hemorrhagic cases. Complete the full course even if the fish appears well - incomplete courses are the primary driver of Aeromonas treatment failure and resistance.

Step 4: Water Quality Management

Aeromonas is an opportunist. Fix the opportunity.

If the infection developed from a water quality event (ammonia spike, nitrite spike, dissolved oxygen crash), correct that event and prevent recurrence. The fish needs functional water quality to mount an immune response alongside the antibiotics.

Target during treatment:

Ammonia: 0 mg/L
Nitrite: 0 mg/L
Temperature: 22–24°C (70–75°F) to support immune function - if you were at 65°F for KHV monitoring, warming slightly during bacterial treatment is appropriate
Salt: 0.3% to reduce osmotic stress and provide mild antibacterial support
Dissolved oxygen: 7+ mg/L

Step 5: Wound Monitoring

Log the ulcer's size and appearance at every observation. A healing ulcer should show:

Decreasing size (wound contracting)
Cleaner, more defined edges (no expanding hemorrhage)
Pink, granulating tissue at the edges (new skin forming)
No increase in depth

KoiQuanta's wound tracking logs ulcer size numerically and allows photo attachment so you can compare closure progress across days. An ulcer that's the same size at day 10 as day 3 of antibiotic treatment isn't responding - that's when you consider culture/sensitivity testing and alternative antibiotics.

Aeromonas in a Pond vs. Individual Fish

If multiple fish in a pond show Aeromonas signs simultaneously, the infection source is likely environmental - Aeromonas from compromised water quality affecting multiple fish at once.

Treating individual fish without addressing the pond environment is a losing battle. If 3 fish in a 5,000-gallon pond have ulcers, the other 15 are likely subclinically infected or will develop signs soon.

At that point: emergency water testing, large water change, pond-wide salt treatment (0.3%), isolation of visibly affected fish, and consultation with a fish veterinarian about pond-wide antibiotic food delivery.

FAQ

What does Aeromonas infection look like in koi?

The most common presentations are ulcers - open wounds with reddish, inflamed edges and white/dead tissue at the center - and hemorrhagic redness at fin bases and across the body. Advanced cases show the "pinecone" scale lifting of dropsy, which indicates systemic infection. Early signs before ulcers develop include subtle scale lifting over 1–3 scales, minor redness at fin bases, and slightly reduced appetite.

How do I treat Aeromonas in koi?

The core of Aeromonas treatment is systemic antibiotics - oxytetracycline bath at 10–20 mg/L for a 10–14 day minimum course, or enrofloxacin by prescription for resistant cases. Any open ulcers should be topically cleaned and antiseptic-treated at the start of antibiotic therapy. Maintain 0.3% salt, optimal water quality (zero ammonia and nitrite), and 22–24°C temperature during treatment to support immune function alongside the antibiotics.

Is Aeromonas in koi contagious to other fish?

Yes. Aeromonas is shed from infected fish into the water and can infect other fish through skin abrasions, wounds, or compromised gill tissue. A fish with an open ulcer should be isolated from the main pond or tank immediately. In pond-wide events affecting multiple fish, environmental triggers (water quality deterioration) are usually the source - addressing the water quality is as important as treating individual fish.

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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