Yes, food is necessary, but it differs from coral to coral. Star Polyps and Leathers generally need little additional feeding, but Phytoplankton is a good option. Mushrooms prefer a little more meatier fare. Stuff like Mysid Shrimp and Vitamin Enriched Brine Shrimp will be more ideal. Regardless, feeding should be done, but not enough to jeopordize your tanks water quality.