How do you remove tub ring on fiberglass tub?

Two things come to mind. First a scouring powder such as comet or dutchboy work fairly well. Second, a good cleaner, for most things, happens to be a paste made from baking soda and water. It makes a good metal polish, or toothpaste in a pinch too. Baking powder won't scratch a surface like harsh cleaners might. I would not suggest using ammonia however, I tried that once and it damaged the surface of my tub.