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 or
oodinium. 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>>. <<
Hyposalinity >>would be the best broad approach unless properly able to identify the parasite. Any additional info?
Cheers
Steve