There are two different issues commonly lumped together as HITH/HLLE.
HITH: parasite (Hexamita spp. and/or Spironucleus vortens) that infects discus and angels, is treatable with metronidazole, and is characterized by narrow, deep holes on the head that may include white discharge. It may be able to infect other species, but angels and discus are the ones it is most common in.
HLLE: non-parasite based illness characterized by shallow, wide pits on the head and along the lateral line, common to oscars and many other fish. HLLE is usually caused by low quality food and water, fix these and the issue goes away. In rare cases it has been shown to be caused by carbon (no evidence associating it with the carbon dust versus the carbon).
This is reference to freshwater cases of HITH and HLLE. In marine aquariums HLLE has been well supported to be a dietary issue.
This is based on years of research including discussing this with dozens of hobbyists with experience with one or the other (or both) and comparing what species, the symptoms, successful and failed treatments, etc., as well as primary research articles. The article I wrote on this was accepted by Tropical Fish Hobbyist magazine and is awaiting publication. The reason there is so much debate and conflicting information is because EVERYONE lumps the two issues together. This means that one person will say 'no, my discus was cured when I treated it with metro so it is a parasite', and the other guy says 'no, my oscar didn't react to metro at all, it wasn't until I improved food and water quality that it went away'.
I did not have many, if any, cases come up in rams so I can't say for sure which it was or more likely to be.
