The reason there is no exact cause is because stress brings it out, and stress can come in many forms.
I agree, stress is almost always the underlying cause of HITH, and stress can come in many forms.
I posted the following last year to another member that was having HITH issues with a new fish that he had purchased...........
While the jury is still out on much of the information surrounding the causes of HITH (freshwater) & HLLE (marine) it is clear that there can be several triggers, and what one hobbyist may experience with their fish, may have nothing to do with what the next person experiences with their fish. A good place to get started with understanding this disease & its various triggers can be found on wikipedia. (go figure)
http://en.wikipedia.org/wiki/Head_and_lateral_line_erosion
Treating for hexamita via 3% epsom salt infused food is a very good idea when HITH is suspected. It's safe, cheap, and will remove any possibility of the fish carrying hexamita/spironuleus, as in some cases these internal protozoa that are very common in tropical fish, will explode in numbers in a fish that's under stress. If nothing else it will clean the fish out & remove the possibilty of hex/spiro later surfacing as a secondary infection. If you read the link above you'll see that hexamita & HITH are often associated with one another.
There is also some more current info available on the carbon dust theory as well.
http://www.coralmagazine-us.com/content/activated-carbon-hlle-smoking-gun-found
As far as nutrition and/or vitamins being a cause, or a potential cure, I know of only 2 studies that have involved HLLE and diet, the one linked to above that was performed at the Toledo Zoo, and the one that author mentions by Tilghman et al from the University of Florida. I'm familiar with both studies and in both cases NLS was used as part of the control diet as it has been proven to NOT induce HLLE. In other words, if one wants to remove a nutrient poor diet from the equation it's as simple as feeding NLS. That won't solve or remove the various other potential causes, but it will remove any concerns with regards to insufficient nutrient levels.
