HITH or HLLE (head and lateral line erosion) is as much a symptom as a solitary disease, much as is bloat or edema.
Common in large cichlids such as Oscars and a number of relatives, plus in tangs and a number of SW fish, it most likely represents a meeting of several problems. The big ones are inadequate diet/nutrition/vitamins (especially from the use of feeder fish), less than high water quality (easy with messy predators- they are mess generators), and the reduced immune response available to the fish from poor nurtition, high nitrates, etc., allowing normally present but easly resisted bacterial and protozoan species to establish infections.
HITH/HLLE is routinely cleared by massive cleanup of the tank - reducing substrate depth to a minimum and vacuuming that fully every week, maintaining high water quality by partials sufficient to hold nitrate below 20 ppm, and not over-feeding the fish - including cutting out all feeder fish from the diet (which may take some serious retraining with a spoiled fish). All the antibiotics and anti-protozoan agents in the world will not cure it if the causes are not corrected. Nor will changing from any given filter type to another specic filter - that is snake oil. General tank upkeep and quality food are the the keys. If the causes are corrected, it is as likely as not to clear without medication.
HTH