How many QT new or sick fish for ich?

[tang daddy]

I have gotten Ich in my reef tank a few times but with pristine conditions the fish never passes it on and they shake it within 3 days max.

My fowler however hasn't had that luck with bigger fish I add, they seem to pass it to the other fish like a marathon runner passing a baton. however If I don't add sick fish, once they are all well no problems.