I think one of my Boseman's Rainbows is sick

.
Tomorrow (Thursday) I'll have had him (and the others) for one week.
The reason I think he may be sick is because he is just hanging at the top of the tank alone and he didn't seem to want to eat. (normally he is like a pig with fins) He dosen't seem to be breathing heavy, and I didn't observe any outward physical signs of illness (no white spots, puffy stomach, cloudy eyes etc)
He appeared fine when I first got home from work.
I had tested my water this evening and my ammonia was 0, nitrites were 0 and nitrates were 10 (which is the same as my tap water). The other two Boseman's appear to be fine.
Any thoughts?