One huge misconception is that tangs are strictly herbivores - they are not. They eat a lot of pods or other organisms when they graze the algae on live rocks. If you are only feeding him nori, he is not going to gain much weight. He will need meaty food like mysis. Pellets would be best if he takes it but it doesn't sound like he does - something you need to train him on. My powder blue tang is crazy fat because he eats everything