Having recently dealt with ich in my DT, I am now a firm believer in a few things:
- put the new critters into a QT unless they are the kind which cannot carry ich in any fashion (my cleaner shrimp went straight into the DT)
- get a serious parasite destruction crew (I just picked up a couple ORA cleaner gobies - cool little fish and completely useful)
- pay attention and learn everything you can from RC and your LFS
Only one fish survived the ich in my DT, and that's the one (powder brown tang) who contracted it first. That fish survived because I quickly put it into a QT and lowered the salinity over the course of a few days to just above brackish levels.
Once it's in there, the ich is in there. While it needs a fish host to survive through all the stages of its life cycle, it also takes a long time fallow to truly remove it.
Rather than believe that 10 week fallow cycle actually killed off every last bit of those little suckers, I went with the method of adding some critters who actively kill it off.
IMO, it's all part of balancing the tank out nicely. There are lots of cool creatures that work well together to help keep our reefs functioning well. Learn about them, employ them and build a happy reef that doesn't keep you up at night wondering what's going to go wrong next.