What causes hole in the head disease?

The precise cause isn't really identified. There are several suspects, from poor water quality, malnutrition, parasites, and bacteria all having strong support. It is common to many fish--large cichlids, and SW tangs in particular. Since these fish are often fed inadequate diets, I suspect that the cause is a combination of things, with diet factoring in heavily.

The clinical signs are pin-sized holes developing along the head and lateral line of the fish. These holes eventually grow in size, and can become secondarily infected. Some fish can recover and the holes fill in--often though, the damage is too severe and the fish will always have scarring from the holes.

Best advice I can give--make sure you provide an appropriate, varied diet for your fish, and make sure water quality is top-notch.
 
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