One of the reasons that marine ich can exist in apparently healthy aquarium is due to fish immunity. Quote from a publication from University of Florida:
“Fish that survive a Cryptocaryon infection develop immunity, which can prevent significant disease for up to
6 months (Burgess 1992; Burgess and Matthews 1995). However, these survivors may act as carriers and provide a reservoir for future outbreaks (Colorni and Burgess 1997).”
There also can be a continuous small infection on fish but fish is appeared to be healthy due to strong immune system.
The time for ich to dormant is vary dramatically, from 3 to 72 days (Colorni and Burgess 1997). That rendered the common two to three week QT practice from some retailers ineffective.
Hope these helps.
Sent from my iPhone using Tapatalk