There are a lot of things we need to consider when addressing this issue. First, what are your water parameters? The water sees a little cloudy in the picture you posted. Second, how big is the patch? If you go to the Freshwater & Brackish - Unhealthy Fish section of this forum, you'll find a post with a list of diseases and their symptoms. That should help in diagnosing the problem. I'm going there now to see if I can find you any help. The last thing I wanted to ask is if your tank has been properly cycled? If you don't know what that is or how to do it there is plenty of info on this site and loads of people that are willing to help.