I agree about the damsels bullying and possibly stressing the tangs to death and this could be why you are seeing simptoms of ich on the yellow get rid of the damsels
on a side note tangs are very sensitive to things that can stress them I lost a powder brown by the simple act of adding sand to my tank it didn't like the color change and up and died on me
also use extreem caution when introducing new tangs with established ones as they can be very aggresive toward each other in these situations
marc
