I actually have documented two cases of HLLE in fish in the wild: a juvenile emporer angel in a black sand area and a black longnose tang. The latter was photographed with a strong flash, so that made it stand out more, but the emperor was a classic case. I suspect the volcanic black sand may have been the cause.Although carbon may or may not be the only cause, captivity is the key. Fish in the sea don't get it. But of course we don't use carbon in the sea.
Jay