I've loaded absolutely mountains of wild coral colonies, maricultured and aquacultured frags, and softies, from many sources, into my DT over the last 1.5 years, which contains 22 assorted fish from blennies to wrasses. I give each coral a 2 minute dip in CoralRx for softies and lps, and in Bayer for sps. Otherwise no coral QT. I've not seen a single fish illness during this time. I think the probability of transmission of fish diseases this way is remote. I've also had no coral illnesses or predators, although the risk here of not doing QT is admittedly higher. I have seen nuisance algae pop up, presumably from spores, after just a coral dip.
OP, I think your fish may well have brought whatever this is over from the QT, as the sequence of events you describe strongly suggests. Crypto and velvet shouldn't have survived that copper. I thought I knew what brook is supposed to look like but it sounds like it's more variable than I thought.
I'm soon going to purchase a harem of clowns; I think I will have to question the supplier closely on this.
