Likely to be spread by yr new PBT. The powdered series are super duper ich magnet compared to other tangs. New tanks, tanks with residents aggressive tangs or those with poor/avg water parameters shd avoid.
Also, if yr tank water parameters are not ideal, the rest of the fishes will get stressed and easily attacked by those nasty ich parasites.
Keeping yr water at optimal level is impt for you now and don't add anymore LS which will introduce possible new parasites and increase bioload. Garlic soaked fd and water change will help.