mis management of the nitrogen cycle is said to contribute to hole in the head desease . Maybe you could step up the water changes [ not to insult your maintanence practices ]. This was my case a few years ago . I started doing larger and more frequent changes , and feeding a little less , than I was doing and slowly the infected fish became well . this has been my experience .
Is it just the new guy still , he may have been subject to poor water quality[ most likely ] maybe you could set him up by himself for a while [ if you haven't already ], remove any food he doesn't eat right away , if any , and any waist he my create as soon as you see it as well[ overkill and easier said than done ]. There are meds , but I found that fresh water really helped my cichlids .
