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You can feed them many things such as flake foods, spirulina flake, frozen foods, pellets, and seaweed veggies such as nori for the tangs to graze on. Tangs like to pick at food all day, and it's good to have some seaweed hanging on a clip for them to pick on for the most part of the day. All the fish should eat the flake type and frozen food. You can also make your own food!
As the others have said a variety is the key. I feed flake, mysis and vitamin fortified brine every other day and put some seaweed selects on a clip on the other days for the tangs.