Ryan when you write Hexamita, do you mean "hole in the head"? Surely that would be visible?
Also Metronidazole cannot be bought over the counter here and discusnoobie has already been denied fish drugs by a vet by the sounds of it.
When I first bought 5 young discus, 3 of them did exactly the same.. went dark and was sulking in the corner until in the end lying on the bottom and eventually died... I put it down to stress and/or low quality inbred fish.