True fish can aquire a short term
immunity to ich but it does not remian so indefinately. Typically most stressful situations will bring about the onset of the parasite as the fishs' immune system is lowered but only if
the parasite is present in the first place. It does not just appear from nowhere.
One of the great benefits of a QT
is the fish is in a controlled environment where it can receive treatment immediately. The problem with automatically treating for ich is the fish may have something completely different and often meds cannot be mixed and one type must be removed before the proper med needed can be started. This can dramatically increase the time it takes to properly treat the fish and lead to it's loss.