Let me explain a little from the perpective of a vendor.
Fish imported are under tremendous stress. They are collected in a very tight space which they are not used to; they scared and water holding become polluted as their own waste cannot get detoxified.
The first stop they get to is a collection center, good healthy fish or bad sick fish. Water in the center may get better again but not for long, these fish are on route to the wholesaler.
At this time, fish will be in water that are mostly copper added, of course lightly.
Fish get ship in a cargo cabin where it is not pressurized.
Fish get in the tanks of whoelsalers. Most wholesalers, if not all, use copper in their water.
So far, if the fish has infectious dieases, it will not infect other fish. But light copper does not cure their illness. It just prevent most of the diease to spread.
Fish fly again unpressurized to the vendors' tank.
If it's a retail, most likely the volume of the fish does not warrant them enough space to QT.
So, most, not all, LFS would just let the fish in their system.
Since they are LFS, they cannot put copper in their water or the whole filter part will die and they have a hell of time to contiuously do water change.
Well, if a fish is infected before, and that after all the stress, its illness will manifest, ten times , zillion times more severely.
So if a LFS has a good record of keeping their tank free of diease, the fish come from there is a lot safer and live longer. However, if the fish come from a LFS which has a low record of healthy tank, the fish has a big chance of getting infected and die soon.
For one very common store PETXX we visit at Union Square, their clown fish come from ORA and many vendors here get their clowns from ORA too. But the survival rate of the same sourced fish are VERY different.