Too late for this to really help the current situation but, even if you don't have a quarantine tank, the affected fish should have been isolated as soon as you first saw spots. Isolation can be as simple as floating a small container in the tank with a small air stone. The reason I suggest floating a container in the tank is that it will keep the water temperature in the quarantine constant. Unless things have changed since I last had a tank, long time ago, you should not have any substrat in the quarantine.
I am VERY sorry for your loss!
I don't know if this is still considered true but it was 25 years ago to be consider a good preventive measure. While not a total prevention it was considered that keeping a tank above 79 degrees F would make it harder for Ich to establish. Like I said, I don't know if this is still considered true but I still keep my current tank at ~81 degrees.