So think of the very best aquarist you know, I guarantee he will miss disease in observational QT. The problem is fish can carry spores or latent infections that become active weeks later. None of this is visible. Beyond that the fish may harbor parasites in their gills, which is completely invisible to our observation. If you are going to the trouble of setting up a QT tank then adding medication is a very small step. CP is a great medication that works very very well, however not accessible to most and not all fish can tolerate it. Copper is the next best broad treatment that helps eliminate external parasites. One study showed 80% ornamental fish had internal parasites. Critical to treat for that as well.