Here are some things to look for to see if the fish you want to purchase are unhealthy: raised scales, a swollen abdomen, bulging eyeballs, an enlarged head, ulcers, lack of movement, loss of balance, damaged fins and scales, fungus growths, or the fish is continuously scratching itself against rocks or other objects.
A few things to look for in healthy fish are: a lively disposition, erect fins, bright colors, good balance when swimming and a good appetite.