To really be able to diagnose the issue, pictures and video are important. Background information about the fish and the tank is also needed. This post explains that:
Aquarium Fish Disease Problem Solving Determining the best course of action for a problem with your aquarium can be a difficult task. Even advanced aquarists will often need to seek the help of others for complicated or unique issues. Our intent here on Reef 2 Reef is to offer expert advice...