feeding seaweed

I've fed various brands and they all made my tang really fat. I mostly fed the green that you can get at the grocery store and mixed in red or brown for variety.

 

go to an Asian market and buy unseasoned Nori and the red Gracilaria.  Have used both for years and tangs LOVE it.  Maybe some grocery stores carry it now but be sure it is unseasoned.  It's so much cheaper and probably better than the ones sold in fish stores.  You can also buy shrimp, silverslides and squid for any fish that eats them as long as they are frozen and unseasoned or processed.
The Nori also works great if you have a ton of snails and run out of stuff in your tank for them to eat.
I would just rubberband a rolled up piece (size based on your tank stock) to a rock and drop it in.  They take care of it pretty fast.