I don't think is microbial, but I guess anything is possible. The fish look great, they eat and have the proper posture and breathing rate. They just got shy all of a sudden, and occaisonally skittish.
I think, as Rick mentioned, it is a non-disease issue. I put a piece of driftwood (a big one) back in the tank and am waiting to see if they may have over reacted to that in a delayed manner... it didn't seem to bother them at all when I removed it two or three weeks ago.