Agreed with above. Tangs are very probe to ich. Especially any of the blue variety.

If you want to rid your system of it fishless for 12-16 weeks is the way to go.

Or maintaining a healthy tank will keep your fish alive. My powder blue will show signs of ich just by looking at him wrong. But keeping parameters stable. Garlic enriched foods and plenty of seaweed will keep them healthy enough to fight it off.