These topics would a good place to start to learn about your problem, and what to do about it.
https://forums.saltwaterfish.com/wiki/ich-cryptocaryon-irritans
https://forums.saltwaterfish.com/wiki/quarantine-or-hospital-tank
https://forums.saltwaterfish.com/wiki/hyposalinity
Basically, read through these topics in order and you should get a good handle on what needs to be done. Its a lot of work, but there really is little choice.
Having a quarantine tank as a hobbyist is about as important as having salt mix for your water. Adding new fish wo QT first is a ticking bomb that will go off.