I just posted this drivel on a similar question, too lazy to do it over:
Turnip greens, softened in the freezer, secured to a piece of rock with a rubberband. Old fashioned, I know. But it doesn't fall apart, is very nutritious, and fish love it. Before the "its not from saltwater" comments start: fresh produce was (and still is) used for years before the nori products were on the scene. My tangs love cucumber and broccoli. Probably the most common ingredient in SW herbivore foods is spirulina, and most comes from FW . PE mysis, possibly the most popular frozen food comes from FW as well. Spirulina pellets are great too. As long as its all eaten, I don't think you can overfeed greens to herbivores, there just isn't the calories or nutrition that carnivore foods have; which would explain why tangs spend all day eating on the reef