If your grouper is established in the tank, he will not get ick. Don't worry about that, The tang will fight it off if he is eating and will be fine. Adding some garlic can help. The worst thing to do is stress the fish out any further, trying to catch him will do just that, and it will get worse. I have never emptied a tank due to ick and I never will. If you have good water quality and healthy fish they will never be phased by ick in the system. I have a large tank and house many fish considered to be ick magnets, I have had ick in the system and have had many many fish newly introduced to the tank get a few spots and clear up after a few days to a week and be fine. None of the established fish ever get it from the new fish. The easiest thing to do to avoid new fish getting ick is maintain good water quality and not allow new fish to get picked on when first introduced in the tank. If you can do that, you will never have to fear ick in a system.
