convict tang

Hi all, so I work at my university's wet lab. We have a 250 gal tank with two convict tangs, a snowflake eel, a toby, and a butterflyfish. Today one of the convict tangs had this weird brown spot near its tail. Any help would be greatly appreciated. I have looked around to try and figure it out...

so I’ve got a snowflake eel and a yellow headed/fimbriated eel, and I was wondering what (if anything) I could put with them in a 75 fowlr tank? I’ve already got a small convict tang in a smaller tank that I’m gonna move to the 75 when it’s no longer bite sized. Thanks :):)