According to your profile you have a 30g tank. IMO
, that tank is too small for a yellow tang, tangs are strong swimming fish that require 48" of tank length. I also think you added too many fish to a brand new aquarium. What are your water parameters?
What do you use for filtration?
How did you acclimate the fish?
I think the tang is very stressed and is showing the first signs of parasitic infestation, this would indicate the use of copper for treatment, but you have LR
in the tank. The copper will effectively kill the life on the rock and in the sand. All the fish should be removed to a quarantine/hospital tank and treated with either copper (at a level of .15-.20ppm for 3 weeks) or hyposalinity
, leaving the main tank fishless for 4-6 weeks.