I've got a friend who works at a public aquarium. Regranex is a miracle drug for this, but its prohibitively expensive, and mentioned in the article.
My thoughts on HLLE is this: It doesnt have any 1 specific cause. Its just a very sensitive area, and is kind of like the canary in the coal mine: Its the first place Tangs seem to show symptoms when theyre having systematic troubles.
As far as the carbon issue, the aquarium my friend works at, had a carbon canister break, and the carbon got chewed up by the pump. All the fish came down with HLLE. As soon as they moved fish out, they woudl get better. When they figured out that the canister was broken, they repaired it, and the issues went away.