There are several things to consider when selecting a new fish. Assuming that you have already selected the species of fish, you must mainly consider the health of the fish. There are two main components in determining the health of a fish, appearance and behavior. The following factors should be considered by appearance:

• Fins - The fins of the fish should be fully extended and not frayed. If the fins are frayed, the fish may be fighting a bacterial infection.
  

• Body Surface - There should be no abnormal spots or soars on the exterior of the fish's body. In addition, check for indications of parasites on the fish. Unless the fish is light in color, you should be able to see Amyloodinium cysts on the fish's exterior. These will appear as tiny white specks.
  

• Abdomen - Occasionally, a fish will be malnurished. Look at the abdomen of the fish. It should not be sunken in at all. Rather, the abdomen should appear slightly rounded.
  

• Eyes - The eyes of a new fish should be clear and not protruding. If a fish's eyes are cloudy or "popping out" slightly, the fish is not in good health, and has a bacterial infection. Note that is some species such as grouper, the eyes of the fish can temporarily cloud somewhat when fed live food, but this shouldn't persist for more than a day or so. Otherwise something is wrong.
  

Behavior is also an important component in determining the health of a new fish. The following should be taken into consideration regarding behavior:

• Breathing - The breathing of a new fish should be moderate and relaxed. A fish which appears to be breathing rapidly is in bad condition and is under potentially lethal stress.
  

• Feeding - Too often people take a fish home without ever establishing that it is eating. Ask the seller to feed the fish. This way you will know that the fish should eat when you get it home. Remember that when introducing a new fish to your aquarium that it is temporarily stressed, and may not eat for a day or two. Eventually it will eat again, so be patient at first.