I would encourage you to please edit this factually incorrect statement, especially since you carry a Reef Squad and other important person badges.
Ich (
Cryptocaryon irritans) is a an organism, specifically, a protist. A fish parasite. Block the organism from entering your tank, and it will never exist in your tank.
It does not appear spontaneously. It is not a byproduct of some biological function such as "stress".
There are multiple effective ways to kill the parasite and prevent transmission from an infected fish to uninfected fish, i.e., quarantine and therapeutic treatment (copper, drugs) or halting the parasite life cycle, which has interruptible stages (TTM). There is the possibility of "resistant" or aberrant parasite strains surviving therapeutic treatment/TTM, but it seems that the likelihood of that happening for the hobbyist is very small--maybe
@Jay Hemdal has actual numbers?
The parasite may also hitch a ride on hard substrates such as coral skeletons. This happens during a specific stage of the parasite life cycle. While possible to get ich into your tank this way, it is not an absolute eventuality at the hobbyist level, and people can also quarantine coral. You can also buy quarantined coral, e.g. SPS Tank (
@Dooly 's operation).
There are cautious reefers in my area that have the opposite experience and have effectively prevented ich and other parasites from entering their systems for years. I am aiming for an ich-free reefing journey. Ultimately, that statement is false and can lead a hobbyist that otherwise might decide to take a cautious approach to taking a riskier approach. It's the same as saying "Syphilis will get into you no matter what you do". So why bother giving penicillin to people with syphilis or wearing silly things like condoms?