Causative agent: Flavobacterium columnare — gram-negative rod bacterium
Columnaris disease, caused by the gram-negative bacterium Flavobacterium columnare, is one of the most commonly misidentified diseases in Australian freshwater aquariums. Its white, cotton-like growths around the mouth and body are often mistaken for a fungal infection — but columnaris is bacterial and requires antibiotic treatment to resolve. The bacteria thrives in warm water above 25°C, making Australian tropical fish tanks particularly vulnerable during summer months. The distinctive 'saddleback' lesion that can form across the dorsal region of a fish is a classic sign of an advanced infection. Poor water quality and stress are the primary triggers, so addressing the root cause is just as important as medication. Aristopet Aquaricycline and Aquasonic Bio-Tet are effective antibiotic treatments available at Australian aquarium stores. The disease can spread rapidly through a community tank, so isolating affected fish and treating the whole tank promptly is essential to preventing losses.
Symptoms
- •White or grey cottony patches around mouth, body, and fins
- •frayed fins
- •saddle-shaped lesion across back
- •clamped fins
- •rapid breathing
- •loss of appetite
Treatment
- •Broad-spectrum antibiotics (erythromycin, minocycline, tetracycline, kanamycin)
- •salt baths
- •potassium permanganate dips
- •improve water quality immediately
Australian Medications
- •Aristopet Aquaricycline (tetracycline)
- •Aquasonic Bio-Tet
- •Aquasonic Bactonex
Commonly Affected Fish
- •All freshwater fish
- •livebearers (guppies, mollies), catfish, tetras, goldfish particularly susceptible
Important Notes
Thrives above 25°C. Often mistaken for fungal infection — it requires antibiotics, not antifungal medication. Address water quality as a priority.