Ich!!!

hey guys. I don't get it. I QT all my fish always and my tank has been spotless for a while, water params are SG: 1.0264, Amm: 0, Nitrite: 0, Nitrate: <5ppm, Phosphate: <0.1ppm, Alk: 9, Mg: 1400... I test with salifert, refractometer and an Ion Chromatograph in the lab...

I just dont get it, but I have ich. My clowns showed signs of it first, caught it early today. Now my yellow tang is itching up against things... I hope its not Brook..... but anyway I have no friggin clue why I am getting ich if my water is near perfect and I do 20% water changes every week, 40% monthly, wet skim, clean my filter literally daily, and had healthy QT'ed fish that were healthy for a long time in my DT.... I am at a loss here. Please help. WTH do I do? something with no copper? QT the fish and treat with copper? I am so sick of this and I thought I'd be over it... my fish in the past have gotten it and I just kept up with clean water and it went away... I just don't understand why they would be stressed to the point of disease when everything in my tank is sparkling and water chems are great. I am so mad right now. Please help. This on top of my alcoholic neighbor p*ssing me off, I have very little patience.