Freshwater Tropical · Goldfish · Koi & Pond
Aeromonas (Ulcer Disease)
Bacterial infection causing open ulcers, hemorrhagic patches, and red sores. The #1 cause of mortality in koi ponds during spring and summer.
Severity: Critical
*Aeromonas hydrophila* and related species cause some of the most damaging bacterial infections in ornamental fish. Lesions appear as ulcers with red rims, often near the dorsal fin or flanks. The pathogen lives in the fish’s bloodstream — surface treatment alone is not enough.
Causes
Stress, poor water quality, and skin damage are the primary precipitants. Spring outbreaks in koi ponds are common as water warms and fish immune systems lag behind bacterial replication.
Treatment
Aeromonas requires a **two-pronged approach**: an antibiotic in the food (kanamycin, oxytetracycline) to reach the bloodstream, *and* a water-borne treatment for skin colonisation. Treat for 10–14 days. Healing of severe wounds may take a full month. **Do not salt the pond** — open wounds in salted water often kill the fish from osmotic stress.
Recommended medications
Antibiotic
Koi Fix For Food
Medicated food additive for koi with bacterial ulcers and open sores. Reaches the bloodstream — water treatments cannot.
from $54.00
Antibiotic
Koi Fix For Water
Water-borne version of Koi Fix — use when the fish are too sick to eat. Targets surface bacterial…
from $48.00
Antibiotic
Nitrofuracin Green
Wide-spectrum antibacterial and antifungal. Excellent for new fish, ammonia burn, transport stress, and quarantine.
from $32.00
Recommended equipment
Hardware that prevents, monitors, or treats this condition — listed with the most important first.
Aquaculture
Pond Aeration System
Dissolved oxygen management in large outdoor systems
from $200–$2000
View details →
Aquaculture
Fish Grader / Sorter
Overcrowding stress and size-based aggression causing wounds
from $2000–$6000
View details →
Sterilization
Ozone Generator
Bacteria, water clarity, pathogen control
from $80–$500
View details →Prevention
Stable water parameters, low stocking density, and quarantine of all new fish. Pond sediment vacuuming during spring reduces bacterial load.
Frequently asked questions
Should I salt my koi pond if I see ulcers?
Adding salt above 0.1% to a pond with ulcered fish is one of the fastest ways to lose them. Treat with antibiotics in the food and water instead.
How long do Aeromonas ulcers take to heal?
With proper treatment, surface healing begins within 7–10 days. Deep ulcers can take 4–6 weeks and may leave permanent scars.