If you look at what you buy why QT? If the thing looks not healthy it is not healthy.
It appears either you are very lucky or are relatively new to all of this. Parasitic, bacterial, and viral infections do not have to show a physical manifestation. The reason most of us QT is because some pathogen wrecked havoc in our DT ,and we lost many fish. NO ONE enjoys QT their fish, but we are trying our best to help keep healthy fish