The red "rash" area is most likley a mild bacterial infection. These can often be cleared up by a few decent water changes. I would do about 3-4 10-15% changes over the next 2 weeks. The tang also looks like it may have the onset of HLLE
(although it does look plump
) so make sure you fortify it's food with some Selcon and ensure it gets lots of spurlina based herbavor foods.
If the red streaking does not improve after a few water changes and the inproved feedings you may need to quarantine
and treat with an antibiotic. Be sure not
to use antibiotics in the main tank. You will destroy the bio filter.