Nori daily if you can they love it and it’s so good for them. I try to give mine every day but it’s a bit cumbersome if I’m not around. Tangs are active so feeding more often is beneficial but not practical as we all have lives, well most of us lol just give them a varied diet and try to avoid cheap pellets. I think twice a day would be more than plenty and make sure they are eating all of what ever you place in the tank as with any fish.