There are various diseases that fish get but the main problem with most fish is stress. All fish have stress. Your fish will probably have a stoke if its fishes stress level gets to high, if you think this might be happening check with your local veterinarian as soon as possible. Different fish may need different foods so check with the pet store owner were you get the fish about what would be best for the breed your buying.
When you walk into a tropical fish store with the intention of selecting fish for your aquarium, you have a lot to think about. What fish do I want? Is my tank the proper size? Do I have sufficient filtration? Is the tank in a suitable location? Do I have enough light? These and other questions are important and need to be answered. So make sure you do the research and answer these questions before making your selection. You want to make your fishes life as pleasurable as possible.
Make your fishes' tank as much like home as possible, turn it into a reef. Include plants and other objects native to your fishes' home. Your fish will love all the stuff that you put into its new domesticated environment, away from home.