Man.....I'm so sorry. I have been in your shoes. Not knowing what I did wrong or what I could of done to prevent it. It really.....REALLY sucks. Made me want to hang up the towel and everything. But you live and learn. Big key is to NOT dwell on it. I did so much so that I started getting paranoid and made foolish decisions. Thinking "well when I didn't quarantine I didn't have these problems" or "maybe I should try this instead of that".
I'm thinking you just had one bad fish that ruined the bunch? The unknown contaminate is possible. Sometimes we focus so hard on the more difficult things that we over- look the basics and obvious. Stupid things like spending $$$ on food because my bandit wasn't eating only to realize my tank temp is set at 78 degrees. Or (and this is going back) trying to figure out why my fish are breathing so rapidly only to realize I filled the tank up too much after a WC to my QT.
The food thing is....very highly unlikely. I've been in the hobby for 8 years now, running a LFS for a few of those years as well, and it'd be the first time I've ever heard of that happening. Especially coming from such a reputable vendor.
I am not sure if Brian runs copper in his FO system or not TBH. All the tanks, however, are set up BB with PVC. So I suppose it is possible. Just ask him. A lot of stores do it though.....
Keep your head up man....