[Blueface]

BTW,
Some fish can fight it well and in time, it all goes away.
Some fish need lots of manual help.

When you see the cysts on them, that is because their tolerance to fight is down or gone. When their slime coats are fine, no ich will affect them as the larvea will not be able to penetrate the coat to feed and grow into a new cysts. That is why some fish just don't seem to get it or if they do, it eventually goes away after only minimal amounts.
When their immunity is down, the slime coat does not produce as much and hence the parasites can multiply.
Their slime coats also serve another function most don't know. It helps filter salt, along with their organs (kidney). Salt water fish filter the salt via their organs and slime coat.
When they are stressed and infested, they stress yet more and produce yet less slime coat. Eventually, the gills get attacked very badly and they suffocate.

So......now go back to the salinity. You lower it, it helps them have to filter it less. Their bodies don't need to stress more by not only trying to filter all that salt via organs only and at the same time, you eliminate the cause (the parasites blow up in fresh water).