Seriously,
Crytocaryon kills fish. It can't get in your tank unless you put it there. qaurantining fish and pre treating them prevents it.
It kills many, many ,way too many healthy fish and weak fish. Doesn't really matter much.
There are no effective in reef tank remedies: not garlic;not uv; not extra vitamins; not cinamon; not ginger;, not various commercial concoctions; not cleaner shrimp; not ozone; not potato chips,: not chocolate ice cream;not chicken soup ; nothing.
The choice is clear run a tank with ich or without . In between is just wishful thinking.
After 5 t o10 days or so a fish surviving intial infestaion often appears better because the parasite has had it's fill of fish and leaves to encyst and reproduce itself by 1 or 2 hundred fold for the second attack ,third attack and so on. Here's the math: 1 goes to 200, 200 goes to 40,000, 40000 goes to 8 million Just 3 life cycles or about 6 weeks with food supply limiting that of course. It stops when the fish die or in the sea or tank transfer method just move on. Once it breaks out of the cysts it has only a day to find fish to eat it strarves to death.
Fish sometimes develop partial immunity and resist seond and third attacks. The immunity isn't from a lack of stress or special foods , it's from exposure to the parasite in the first place. The immunity is to the particular strain of ich, there are thousands at least, and the immunity is only partial. Parsites still infest the tender tissue of the gills, nostiils and mouth often unseen and survive in the tank at lower levels just waiting for a non immune fish to be introduced or a minor stress event to occur leaving the partially immune fish more vulnerable. A single strain of ich without cross fertilization from other strains can keep going for years and breakout in a massive infestation at any time.