Freshwater Tropical · Goldfish
Columnaris
A highly contagious bacterial infection (Flavobacterium columnare) presenting as white/grey patches, often on the mouth, fins, or gills. One of the fastest-killing freshwater diseases.
Severity: Severe
Columnaris is caused by *Flavobacterium columnare*, a gram-negative rod bacterium found in almost all freshwater systems. It is opportunistic: outbreaks follow stress events — temperature swings, ammonia spikes, overcrowding, or shipping.
The disease is frequently misidentified as a fungus because of the cotton-like appearance of advanced lesions. It is not a fungus, and treating it as one wastes critical time.
Causes
Stress is the trigger. Common precipitants include rapid temperature changes (especially warming above 24 °C / 75 °F), poor water quality (ammonia, nitrite), bag stress after shipping, aggressive tank-mates, and overcrowding. The bacterium itself is ubiquitous; it is the host's compromised immunity that produces disease.
Treatment
Treat in a hospital tank when possible. Lower the water temperature toward 21 °C (70 °F) — *F. columnare* virulence increases with heat. Reduce the salt concentration: contrary to widespread advice, columnaris reproduces faster in salted water. Treat with a broad-spectrum, water-borne antibiotic such as **Nitrofuracin Green** or a kanamycin/furan-2 combination for 7–10 days. For Koi pond outbreaks, medicated food containing kanamycin or oxytetracycline is more effective than water dosing.
Recommended medications
Antibiotic
Nitrofuracin Green
Wide-spectrum antibacterial and antifungal. Excellent for new fish, ammonia burn, transport stress, and quarantine.
from $32.00
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
Recommended equipment
Hardware that prevents, monitors, or treats this condition — listed with the most important first.
Aquaculture
Fish Grader / Sorter
Overcrowding stress and size-based aggression causing wounds
from $2000–$6000
View details →
Monitoring
Microscope for Fish Health
Definitive parasite and pathogen identification
from $100–$800
View details →
Temperature Control
Aquarium Heater
Ich outbreaks, stress from temperature swings
from $15–$100
View details →
Water Testing
Nitrite / Nitrate Test Kit
Nitrogen cycle imbalance, chronic stress
from $200–$500
View details →
Prevention
Quarantine new arrivals for 14 days. Maintain ammonia and nitrite at 0 ppm. Avoid sudden temperature changes and overstocking. Pre-treat shipping bags arriving from importers with **Nitrofuracin Green** before pouring fish into your system.
Frequently asked questions
Is columnaris a fungus?
No. Despite the cotton-like appearance, columnaris is a bacterial infection (*Flavobacterium columnare*). Antifungal treatments do not work and waste valuable time.
Does salt cure columnaris?
No. Adding salt actually accelerates *F. columnare* growth. Treat with a proper antibiotic instead, and if anything, lower salinity during an outbreak.
How fast does columnaris kill fish?
Aggressive strains can kill a fish within 24 hours of visible symptoms. The chronic form can take 1–3 weeks.