<a href=showthread.php?s=&postid=12517904#post12517904 target=_blank>Originally posted</a> by JHemdal
Folks,
Good picture - IMO this is HLLE, not the most common location to see this on a fish, but emperor angels do get that in that location more often than other fish do. The bilateral symmetry is the clincher. Now - as to the cause, that can't be determined with the information at hand. It is supremely unlikley that this erosion is caused by the angel poking its head into narrow crevices - what, there aren't any narrow crevices out on a reef? (grin) You never see HLLE on wild fish (with the exception of rostratum tangs).
You can run a search here for HLLE, but as I always warn folks, your going to wade through a lot of garbage and bad opinion to get to any real usuable advice. Three "causes" of HLLE can be dismissed - stray voltage, lack of HUFA's in the diet and disease. The remaining three contenders are: carbon dust, unknown irritants and vitamin defficiency. It also may be a multivariate issue.
JHemdal