A healthy fish should swim readily around its aquarium unless it is a bottom-dwelling catfish, of course. The fish shouldn't be having difficulty swimming, if it does it may be suffering from swim-bladder disorder. A fishes eyes should not buldge unless it is a feature of that species. Swollen underparts may be a sign of dropsy, make sure they aren't swollen. Scales should be clean and colorful with no signs of damage. If damage is seen, don't buy that fish as it could be infected from its damaged scales. A fish tail should be resonablly round or square-ish in shape, this indicates it is in good health. Damaged fins could be a sign of an infected fish. Be careful when buying, look closley at every detail on the fish to make sure it is a healthy one. A fishes color shouldn't be dull if it is a colorful fish, if its color is dull and it is a colorful fish, it may be suffering from a poor food diet. Always check your fish everyday to check for signs of illnesses.