Randy I know about the carbon theory but I have a different theory. Only captive fish get HLLE, it is not found in the sea and tangs are most vulnerable. I believe that is because tangs are schooling fish and have a more well developed lateral line system which allows them to "feel" or sense any solid object around them. That allows them to swim inches from another fish in the school without hitting them. In the sea, they don't sense anything unless it is the bottom a rock or another fish. If they are swimming in a school, like they always are, they sense the fish next to them and turn with them. But in a tank, they always sense the glass and bottom. Then can not get away from either of them and their lateral line is constantly bombarded with very close signals that they can't get away from. They also can't see the glass from inside just like we don't see the glass from outside. Their senses are overloaded causing the lateral line to disintegrate which always starts on the head where the line connects to the brain and it continues to the tail. It only invaded other parts when there is no lateral line left. Pelagic Sharks which have a different system are even more sense conscious than real bony fish and can't be kept in square sided tanks at all and must be in a round tank so the signals are scattered. Anyway, that is my theory.