Fungal diseases in ornamental fish are usually external and almost always secondary in nature.  Precipitating factors include disease caused by a different pathogen, poor environmental conditions (water quality), or injury/trauma.  "Fungal" infections are caused by different groups of organisms including true Fungi and Oomycetes ("water molds"). 

Saprolegnia is perhaps the most common "fungal" infection in pet fish, but actually is classified as a water mold (Oomycetes) and not a true fungus.  This organism usually will affect areas of previous injury and causes an external infection.  The presence of external parasitic infections and Columnaris bacteria may be seen with Saprolegnia, therefore wet mounts are needed to determine the proper treatment.  Clinical signs include fluffy white lesions on the skin and fins.  Diagnosis is by wet mount examination revealing the presence of aseptate hyphae and zoosporangia.  Treatment includes environmental management/removal of inciting cause, supportive care and chemicals such as potassium permanganate, formalin, malachite green and salt. 

Branchiomycosis is a "fungal" infection (also classified as a water mold) of the gill tissue by caused by Branchiomyces demigrans and sanguinis.  Clinical signs include increase in opercular rate and gasping for air at the surface.  Gills will appear discolored with pale patchy areas.  Wet mount will reveal damaged gill tissue with fungal hyphae and spores.  Formalin and copper can decrease mortalities but ultimately disinfection and drying of the system(s) is necessary. 

Fusarium is a true fungus that causes disease in pet fish.  It has been described more in marine fish (angelfish, parrotfish, some sharks).  It causes invasive cutaneous lesions and can progress to systemic disease.