Fish can get sick for many reasons. The usual cause is stress caused by a rapid fluctuations in temperature, poor diet, a poor environment in general, or by poor or incorrect water conditions. The 1st thing to do is to determine the cause and then select a suitable course of action. Most health problems cannot be effectively treated until the causes or causes have been eliminated. Check that the temperature of the tank is staying consistent, basic water conditions (ammonia, nitrite, and nitrate) are within proper levels, and how many and which fish are effected. If only one fish is effected it may be a simple problem.
What does the effected fish look like? Are there obvious lumps, spots, cloudy patches, or skin problems? How are/is the effected fish behaving? An accurate description, and a good picture of the effected fish, if possible, can be very helpful when discussing the problem with your retailer or consulting knowledgeable fish keepers like those on our Facebook page. Diagnosing which disease your fish has needs to be done correctly before a course of treatment can be started.
The best place to start would be to check our page of common tropical fish diseases to see if you can determine what it might be from there.
Prevention is always better than having to deal with sick fish so make sure you are providing the correct environment for the fish you are keeping. Is there enough room, the right kind and quantity of hiding spots, the correct substrate, proper lighting, and the correct tank mates? Has the fish been eating well? Are other fish harassing the sick fish, even before it was sick? Answering these questions and the ones discussed earlier will help you accurately diagnose and prevent fish sickness.