I wouldn’t advise adding any powder Blue, powder brown, or similar acanthurus tangs until you remove and treat all fish for ich. They cannot handle ich management. Here’s some reading for you (another article I wrote on the subject matter). https://www.reef2reef.com/ams/ich-and-acanthurus-tangs-years-of-experience-and-ich-management.106/ Also, what you witnessed was a tank boss shift. You don’t want an aggressive fish as tank boss. This is one reason people like adding large naso tangs or larger docile tangs early— it disincentivizes a smaller and much nastier tang from becoming tank boss and terrorizing everyone, they’re effectively kept in check.