Romaine lettuce seems to be a favorite, not only the Yellow Tang, but my Lawnmower Blenny, Cleaner Shrimp, Crabs and Snails devour the pieces I attach to my "lettuce rock" with rubber bands. Broccoli (blanched for a minute or so) is a close second. Alfalfa, spinach and grean beans are not as favorite but are eaten aswell. I change it once a day and that seams to be fine. When I put Nori in, it gets all mushy and eventually disintegrates and makes a mess....