I do not agree with this. There is either ich management or ich eradication. I find that those who QT/Treat fish but don't QT anything wet (corals/inverts) usually find it back in their tank and make the same statement as you did "all systems have ich" If you know the life cycle of c.irritans then you understand that they can be introduced to your DT in their Cyst/tormont stage. I tried ich management at the start when i introduced it to my tank through corals but just didn't work for me. I did TTM (+QT until fallow complete) + Fallow 76 days and 1/2 year on no signs of anything. It is soo relaxing sitting back and enjoying the tank without having to stare at my fish scouting for white spots.