Blue Tang marks on body?

[sham]

Hi can anyone identify these marks on my blue tang. I just noticed them today. I have a purple dottyback that has been a demon and been trying everything to get it out of the tank but no luck so far.

 

[Stv]

Tangs are very prone to ick and disease. Usually stress is the cause and is usually it’s because of tank size. How big is your tank?