Complex sentence: Lionfish are predators of juvenile fish such as, commercially important, grouper and snapper, as well as juvenile parrotfish, which graze on algae in coral reefs preventing the algae from overgrowing and killing corals.
Split into simpler sentences: Lionfish are predators of juvenile fish such as commercially important such as grouper and snapper. Lionfish also eat juvenile parrotfish, which graze on algae and keep it from overgrowing and killing corals.