Too many suppliers and LFS are dosing their fish with sub therapeutic levels of copper and other half measures of meds these days. They do it to keep losses down and fish still looking sellable. Problem is then you or me buy a fish or two from them, and in a few days the copper wears off and velvet, brook, ich, uronema, bacterial infections, etc. comes roaring back, often without warning or the usual symptoms. Not good. Lost a whole tank of fish in QT a few months back. Really sad. The whole tank was gone within 4 days and no visible symptoms until day 3.
Since I started prophylactically treating, starting with copper first and soon, and then General Cure, I haven't lost any fish to disease. Lesson learned.