This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
This disease can affect discus, other cichlids, and many saltwater
fish. The fish develops holes in it's head and sometimes along its
lateral line. Causes are unclear but as in any disease, stress and
poor water quality likely play a role. The Manual of Fish Health
states that HLLE is probably due to nutritional deficiency, especially
of vitamin C. Fish in planted tanks rarely get HLLE, which supports
the nutrition idea, since fish can nibble on the plants and obtain
extra nutrition. Untergasser also observes that the protozoan Hexamita
can be found in the lesions. Untreated cases can eventually prove
disfiguring or fatal.
Remedy: First, make sure water quality is optimal and reduce stress.
Stopping carbon filtration may help as it can remove nutrients from
the water. Then feed a vitamin-enriched food, paying particular
attention to vitamin C supplementation.
For stubborn cases, some books suggest metronidazole (Flagyl) to
eliminate Hexamita (a mildly pathogenic protozoan) from the lesions.
Your mileage may vary with that one. Metrozole and Hex-a-mit are
commercial medications with metronidazole.