Parrotfish are really not fish for the home tank; except in huge tanks with massive filtration and very experienced owners. There are really very few, if any 100% herbivores in the fish world. Even tangs eat many tiny crustaceans while browsing algae. I don't know of any fish that are kept in the home tank without some meaty foods; either frozen or as ingredient in a dry food.
Parrotfish really are an interesting group of fish though; many chomp pieces of live Rock or skeletal coral (with their parrot-like beak), digest what can be digested (algae, tiny animals, etc.), and help the reef by pooping coral sand. Really.