That one BG should represent such a small percentage of your population, I wouldn't think twice. I've seen perfectly health fish (what appeared to be) belly up and die. I currently have one in my 90 gallon aquarium that resembles what you describe. This particular fish does not eat (ever), but constantly uses it's gills to filter micro-organisms. While it continues to gain weight, the head and eyes appear darker than counterparts, and the eyes do buldge.
