Thanks for the article LittleBlueFishlets. I often think this is where I have issues:
farmed fish can also be infected on their gills, meaning that other symptoms are absent
I often wonder if my fish that have been affected in the past, develop an immunity to the parasite, or only harbour the parasite in their gills rather than as a full-blown infection. However, when fry are in the tank, they are much more sensitive and so become infected.
It's so frustrating to deal with. I had three fish arrive recently from the wholesaler with ich, and a few of weeks of heat and treatment and they are back to full health. Ich seems to definitely be a lot easier to cope with.