Quarantining fish is a critical step that most hobbyist do not follow. As mentioned above, a certain level of fortitude is required and I understand why following a strict QT process is the exception and not the rule. I follow a 8 - 10 week process and it takes time to fully stock an empty tank. I currently have five fish with 25 more to go. I have one QT for inverts and three QT available for fish and always have one 10g QT for fish running. It helps for those impulse buys.
FWIW, I quarantine fish purchased from vendors that offer quarantined livestock. Paranoid, absolutely, I have zero patience for disease. Who is responsible for replacing livestock that were infected by a fish that came from a vendor that quarantines livestock?
Also, consider the different vectors that disease can enter a system, such as inverts and sharing of sand.
About 10 years ago, I had a mature 10g anemone and clownfish nano. Ordered a few snails and macro algae to add diversity; I did not quarantine the macro algae or snails. You guessed it, had ich within a week of adding the algae and snails.