Bacteria of the genera Pseudomonas, Aeromonas, Vibrio and others can cause a condition called "dropsy" or swollen abdomen in fantail goldfish and other freshwater species. Symptoms of bacterial infections in goldfish also include red patches on the skin and exophthalmos or bulging eyes. Treatment is more effective in the early stages of the infection and involves the use of antibiotics.
Also called "cotton mould," fungus of the genus Saprolegnia can attack fantail goldfish, causing necrosis on the skin. Affected fish show a cotton-like white layer on the body, which sometimes spreads to the fins. If not treated, Saprolegnia is fatal, as it causes haemodilution, which is a reduction in the number of red blood cells.
Also called anchor worms, copepods are small crustaceans that parasitize several freshwater fish, including the fantail goldfish. Copepods of the genus Lernaea attack the skin and muscles of fantail goldfish in their larval stage, causing inflammation and breathing difficulties when reaching the gills. An infected goldfish often tries to remove the parasites by rubbing its body against anything it finds in the tank.
Protists of the genus Piscinoodinium can attack fantail goldfish, causing the velvet disease. Symptoms include lethargy and the appearance of a velvety brown to golden coat on the back that spreads to the rest of the body and gills, causing death by hypoxia or lack of oxygen. The protist Ichtyopthirius is another common parasite of the fantail goldfish, causing skin lesions on the body and fins and a disease commonly called "white spot."