Do your research long and hard and proactively treat a Achilles with PP and cupramine or chloroquine phosphate to make sure the fish is clear of any and all diseases. If you haven't done what I described above to the current fish you have already in the DT and all fish added to the DT, your Achilles will get ich period. Even if you have never seen any signs of ich on the fish in the DT currently, the DT has it. Achilles are the most beautiful tang there is (IMO) but in order to keep one long term takes perfect water chemistry, high flow and a disease free tank.