Depends from fish to fish, tangs usually are the first to show signs of different diseases, probably because of their thin slime coat and body and high metabolism rate. They spend most of the time grazing on algae and they need a lot of it, and since it is public aquarium you dont want the nasty algae in the tank, so it might be a combination of all of these. Body nature, High metabolism, Lack of Algae, and the previously mentioned issues.
