Branchiomycosis, commonly known as gill rot, is a dangerous fungal disease affecting the gill apparatus of aquarium fish. The causative agents are two species of fungi from the genus Branchiomyces — B. sanguinis and B. demigrans, which differ in morphological characteristics and developmental features. In my experience, this is one of the most insidious diseases capable of destroying a significant portion of aquarium inhabitants within just a few days.
Causes of Branchiomycosis
Fortunately for aquarists, branchiomycosis occurs infrequently, as its development requires serious violations of maintenance conditions. Freshwater fish are most susceptible to infection under poor keeping conditions, while high temperatures and organic debris in the water stimulate disease spread.
The pathogen typically enters home aquariums with live food from natural water bodies. During respiration, fish swallow fungal spores that settle on gill filaments and germinate. The fungus then enters the capillaries of respiratory folds, penetrating veins and connective tissue of the gills.
B. sanguinis localizes in large blood vessels, while B. demigrans develops both inside and outside the gill blood vessels. I strongly recommend paying particular attention to quarantining live food — this is the primary route of infection introduction.
Symptoms of Branchiomycosis
Identifying gill rot by symptoms is challenging, as branchiomycosis resembles signs of other diseases — tuberculosis, ichthyosporidiosis, and dactylogyrosis. This is precisely what makes the disease so treacherous, since lost time threatens mass fish mortality.
Behavioral Changes
First symptoms appear 2-3 days before fish death: they stop taking food, gather near the water surface, and show weak or no response to external stimuli. Due to oxygen starvation, affected fish gasp atmospheric air at the surface and frequently rub their bodies against various aquarium decorations.
Unlike simple oxygen deprivation, fish with branchiomycosis don’t capture air at the surface — their heads remain underwater. This sign helps me distinguish branchiomycosis from ordinary oxygen starvation, which can also lead to aquarium fish shock.
External Signs
A characteristic sign of gill rot is the appearance of dark red streaks, followed by dirty gray spotting and erosion areas on the gills. This “marbled” coloration is very typical for the acute form of branchiomycosis.
Narrowing of blood vessel lumens due to thrombus formation from fungal hyphae impedes blood outflow from gills, causing them to turn almost dark blue. Gill filaments atrophy and significantly decrease in size. At the next disease stage, gill tissue decay and detachment of individual sections occur, sometimes accompanied by saprolegnia development.
In the final stage, fish swim on their side and typically die in this position.
Diagnosing Branchiomycosis
Diagnosis is based on epizootiological data, characteristic disease symptoms, and microscopic examination results of gills from dead fish. Without laboratory analysis, accurately identifying branchiomycosis is extremely difficult.
I always recommend consulting an ichthyopathologist for microscopic examination when this disease is suspected. This disease can destroy up to 70% of aquarium inhabitants in less than a week, making delay unacceptable.
Treatment of Branchiomycosis
Since branchiomycosis progresses very rapidly (4-7 days), applying treatment solutions in the main aquarium is ineffective — treatment must be conducted in a separate vessel, with copper sulfate solution being most effective.
After accurate diagnosis, treatment must begin immediately. All fish should be placed in a quarantine tank with fungicidal medications added. Recommended treatments include Tetra FungiStop, Acrimet, Collargol, Nystatin, table or marine salt, Rivanol, Griseofulvin, Methylene Blue, Phenoxyethanol, and Sera mycopur.
Parallel to treatment, the main aquarium must be thoroughly cleaned of dirt, organic debris, and fish waste. Substrate should be boiled, while the tank itself, equipment, and decorations should be treated with disinfectant solution.
Before returning cured fish to the main aquarium, fill it with dechlorinated water and run aeration at full capacity. I also recommend increasing water flow and replacing at least half the volume with fresh dechlorinated water.
Recovery After Disease
Regeneration of gill filament cells in recovered individuals occurs very slowly and can last 8 to 10 months, but even after this period, gills don’t fully restore — damaged areas remain visible.
In my experience, even completely cured fish require special attention throughout the entire recovery period. It’s essential to maintain ideal water quality, provide enhanced aeration, and avoid any stress factors that might weaken immunity and trigger relapse or secondary infections like columnaris disease.
Prevention of Branchiomycosis
Preventing branchiomycosis is much easier than treating it. Key preventive measures include:
- Strict adherence to hygiene rules and regular aquarium cleaning
- Quarantining all new fish and live food before introducing them to the main tank
- Maintaining optimal water temperature and preventing overheating
- Regular water changes and monitoring organic pollution levels
- Avoiding aquarium overstocking
- Quality filtration and aeration
I strongly recommend beginners study the basics of proper aquarium setup and general principles of disease prevention. Branchiomycosis is a disease of dirty water, and with basic maintenance standards observed, the risk of its occurrence is virtually zero.
Remember: branchiomycosis develops only in aquariums with serious maintenance violations. Clean water, regular maintenance, and careful attention to quarantine procedures are your best defense against this dangerous disease.
Article updated: 2026-07-26
