This can be caused by different things. Can you post all of your water parameters, including ammonia, nitrites, nitrates, pH, salinity and temp? Also, how long has the tank been set up and what all is in it? What size tank? More info would help narrow down the potential cause. Sometimes sub standard water quality can bring this about. Abrasions due to aggression or netting can cause secondary bacterial infections. Often time a large water change will be all that is needed but it's important to identify the cause to make sure you proceed the right way.