The above information is only accurate for freshwater ich. Some people can manage to keep their fish alive as some fish can develop a low level immunity to ich so there is a lower amount of the parasite in their systems (sometimes with the use of UV systems and garlic).
If you have set up your tank properly and do not add any infected fish (usually by quarantining), you will NEVER have ich in your tank. If you do this, your tank will be much healthier as is is hard to eliminate all forms of stress.