You want to dominate your tang's diet with greens (macro-algae, kelp, nori) or your naso tang will slowly wither away. Usually tangs start experiencing head and lateral line erosion(HLLE) if they do not have thier greens. You could get caulerpa from a LFS and grow it in a seperate tank by filling a small tank with the water that you remove from your tank after you do a water change. The water should have some kind of nitrate buildup an will feed the algae. you can also put the algae growing tank outside or in the window to aid in growth. You could put a grow light from home depot which is cheap, but sunlight is cheaper