Hexamita: Hole in the Head Disease
Hole in the head disease is also known as lateral line erosion. Both names describe the symptoms of the disease, which are "holes" or ulcerations on the fish's head or along the lateral line of the fish's head and body. Both saltwater and freshwater fish may be affected.
Hole in the head or lateral line erosion may be caused by viruses or by the protozoan hexamita. There is also speculation that it may be caused by stress, an autoimmune response, and even dietary deficiencies. It is most commonly found in overcrowded tanks with poor water quality, which suggests the role of stress as a primary cause.
If your fish has symptoms of hole in the head disease or lateral line erosion you should first perform a partial water change. Sometimes this disease will improve or disappear if water conditions in the aquarium are improved.
If the cause is hexamita then you should treat the affected fish with metronidazole (Flagyl). You can usually find fish medications that contain metronidazole at most pet stores.
Hexamita is usually introduced into the fish's digestive system through food. The protozoan is sometimes present in small numbers in the digestive tract of fish without causing disease. If large numbers of hexamita are present, it first affects the fish's digestive system and may eventually manifest itself as holes in the head that may extend down the fish's lateral line.
Once the ulcerations or holes form, the fish is susceptible to bacterial and fungal infections. Most research studies on hole in the head disease report the presence of bacteria in the ulcers.
The problem with treating hole in the head is that you generally don't know the cause. It is best not to use fish medications unless absolutely necessary. If you decide to use metronidazole it is best to isolate the affected fish to treat it alone, rather than treating your entire aquarium full of fish.
Freshwater fish that are most susceptible to hole in the head disease are oscars, cichlids, including angelfish, and discus. However, other fish may also get the disease. I've seen it in a dwarf gourami once.
Saltwater fishes most commonly affected are angelfish and surgeonfish, however, other fish may also be affected.