you'll need to catch him and move him to a qt tank. You can treat him with either hypo or copper, garlic will not cure ich only the two mentioned will work for sure. Do not use either in your main tank as they will kill corals or inverts If there are other fish in the tank you will probably have to move them also and leave your tank without fish for 4 to 6 weeks to be sure the ich in the main tank is dead. I like to use cupramine if your planning on using copper its very effective and a gentler form of copper, you will need a test kit to make sure you have the proper level to kill the ich. Follow the directions carefully as copper is toxic to fish in high quanities, the test kit should be able to test for ionic copper api test kit will work also seachem.