DeLgAdO said:a part of that article vitaliy send me was intersting
says that if the ich parasite is not present in the tank, then the fish cannot get the parasite even if the fish has a weakened immune system, and under stress
Scripps did an article back in the early 80s which detailed the organisms present in southern CA drinking water. Ich spores was on the list. So, just doing a simple water change will reintroduce ich to any tank. Keeping a properly heated and healthy environment for your fish will keep their immune systems capable of fighting off outbreaks of the disease.