Good little blurb in Aquarium Fish Magazine (yes, it does exist, and yes, I do read it) about signs of poor fish health.
-A cottonlike material on a fish's mouth could indicate cottonmouth disease, which is contagious.
-Unless a fish is normally pale, paleness indicates unhealthiness.
-Signs of fish parasites include rubbing against aquarium objects, erratic swimming, and white worm infestations
-Red streaks in fins followed by abnormal lightening of fin edges often indicates fin rot, a bacterial infection.