but for white spots, if you quarantine all your fish, then there will not be any white spots in your tank. so no matter how stressed your fish is, they will not get white spots. because there simply isn't any in the tank to begin with. whitespots are not like bacteria where they are found everywhere. they come in through infected fish. so if your tank is white-spotless, then there will be no whitespots regardless of stress factor and water quality.
take this analogy for example.
if a person with a virus is given proper health care and good living conditions, chances of not getting overtaken by the virus is very high.
but if a person infected with the virus is put through stress like over working, over exertion etc, then the virus will take over and eventually make the person sick.
this theory is to support the "dump and hope approach" where living with white spots is possible with good water quality.
now take this other scenario.
if a person without being infected with the virus is put through stress like over working, etc, he will never ever fall sick. why? because he doesn't have the virus in the first place. sure he could suffer from other ailments but he will never suffer from that particular virus infection. why? because the virus does not simply appear out of nowhere.
this is the quarantine theory.
if you quarantine all your fishes for white spots, then they will never ever get white spots because the tank has no white spots. and no matter how stress the fish is, they will never get it because white spots does not appear out of no where.