HITH and HLLE
are two different diseases. HITH is the parasitic form (Hexamita spp. and/or Spironucleus vortens) and this type is treatable with metronidazole. This form is what you will find in angelfish and discus.
is the nonparasitic form found in most other species (including and most commonly in oscars). Since this form is not parasitic it will not respond to meds of any kind. This is the one that requires an improvement in the quality of the food and water. I couldn't agree more with the NLS exclusively and the 10ppm or less nitrate. If thse do not improve the situation take the carbon out of your filter. Carbon causing HLLE
is rare but has happened. It is worth looking into if water and food don't fix it.