I suppliment my useage of ocean nutrition food with market shrimp, I just rip it into pieces and then sqaush it so the fibers of the shrimp sort of seperate and put that in the tank and the fish go to town. I also fine cube wild pacific salmon and drop that in the tank same result the fish go nuts for it. A easy veggie thing to use is nori, the dried seaweed they sell in the grocery store.
Can I eat in your tanks? Yum!
A small caution re: nori/seaweed sheets. Check the ingredient list. Lots of nori have added salt and/or flavoring (ajitsuke=teriyaki flavor). Sushi nori, which comes in large square or rectangular sheets, is usually a safe bet.