What is Branchiomyces?
Branchiomyces demigrans or Gill Rot is caused by the fungi Branchiomyces sanguinis and Branchiomyces demigrans. Both species of fungi are found in fish suffering from an environmental stress, such as low pH (5.8 to 6.5), low dissolved oxygen, or a high algal bloom. Branchiomyces sp. grow at temperatures between 13.9-35°C (57-95°F) but grow best between 25-32.2°C (77-90°F) .
Branchiomyces sp. primarily infect the gill tissue of fish so preventing oxygen takeup. Fish appear lethargic and may be seen gulping air at the water surface. Damaged gills appear marbled with pale areas instead of uniformly bright red. As the dead tissue dies and falls off, the spores are released into the water and transmitted to other fish. High mortalities are often associated with this type of infection.
Good water quality is the best way to control Branchiomycosis. If the disease is discovered, isolate the fish and be careful you do not containment other tanks via water droplets or wet nets, hands, etc.Formalin  and Copper sulphate have been used to help stop deaths. But all tanks and equipment must be disinfected and dried.
- Formalin treatment by Dr. Erik Johnston
- KoiVet - Branchiomyces - by Doctor Erik Johnson