It is an antibiotic, not sure how good it is. You definitely want to stop carbon as it will remove it from the water. Your skimmer will probably go a bit crazy and overflow when you add it. Generally best to follow the directions. Make sure you have plenty of water movement as many meds deplete oxygen.
You need to figure out why your tang has fin issues bacterial infections tend to happen when the immune system is weak. Is it water quality, is he being picked on, do you have a crab or shrimp nipping his fins while he's sleeping, is he stressed because he doesn't have enough space, these and other things can make a fish susceptible to bacterial infections. Always feed high quality marine fish food. With tangs offer sheets of nori regularly and make sure there is vegetable matter in the food. You can also add a vitamin supplement like Selcon to their food.