Do you/did you quarantine your new fish? That is really the only way to avoid it. I am definitely not an expert on fish disease so I'll let others more experienced in that arena chime in, but I know there are a number of avenues you can take. The simplist is to keep feeding your fish to keep him healthy so he can fight off the infection and hope that they both do (and that others don't catch it). But it may not go away on its own, and you may end up needing to remove the infected fish and place them in a quarantine tank for treatment until they are better. A quarantine routine for all new incoming fish is the best (and some will say only) way to avoid disease in your tank. Good luck!