Nori is a nutritious edible sea vegtable (weed). It is the same stuff they use to wrap your sushi in. My fish get nothing but nori, mysis, shrimp, squid, and occasionaly scallops.
My fish are so bright with color its amazing. I must be doing something right.
You can find nori at your local good store such as publix, fresh market or organic store.
I think its cheaper than buying seaweed from your LFS. I buy the emerald cove organci pacific nori and it comes with 6 huge sheets of nori for about 9 bucks as opposed to what the LFS sells you.