Poor water quality or stress can bring the ick back. A healthy happy fish with excellent water quality can have an immune system that sometimes overcome the ick. But it will later return when the fish gets old, stressed or poor water quality. My opinion only not an expert. Tangs are very sensitive to water quality and are often stuck in too small of a tank causing stress. The smaller other fish probably won't get it because they don't get as stressed in an aquarium. JMO