I have had ich twice, and both times it wiped out all my fish, even though I took them out and treated them. It's bloody awful!
Re. fallow period, there's a few articles/threads on here where I've discussed this, but 90 days minimum is the concensus. I did 10 weeks the first time, after QT using hyposalinity, and they had it back within a week
The next time I left it fallow for over 4 months, and so far, no ich. I use cholorquine phosphate for a month to QT the fish, and they also get a 5 min freshwater + myzaxin dip, and a final 6 hour bath in Sera Tremazol and salt water before they go in the DT.
I also QT corals and inverts in another tank. The easy bit about the coral and invert tank is you just dip the corals and put them in, then leave them for 90+ days.