The new fish and the foxface will need to be housed seperately and treated for the time being. By the sounds of it you have either brooklynella
. Either way, any new fish added to the diplay tank will aquire whatever parasite is present.
Even though the new fish may not have a problem, unless you can house them apart from the foxface and not in the main tank, they must all be treated with the foxface in a <<quarantine tank
>>would be the best broad approach unless properly able to identify the parasite. Any additional info?