Ich is almost always present in our systems. It effects fish that are stressed from their environment. Sometimes from moving to a new tank or new tank mates. It can also be caused by changes in water parameters or temperature swings. My Purple Tang had it after he was in my tank for a few days. He showed no signs of ich when I bought him and I have not had a fish with ich in 5-6 years. I added garlic to the food and fed well for about 2 weeks, everything cleared up and he is doing well.