... need a definition of what quarantine is. ...
Does QT automatically mean medication?
Is it just observation?
A great distinction, I'd say that strictly speaking quarantine is isolation for a set time period in order to determine health status.
Some add medication to quarantine. That is a separate issue. As a physician, I understand the debate on antibiotic usage. I'd leave that for another day.
That said, I quarantine everything. For fish, I always use add ich and anti-parasitic therapy. Observation for appearance and behavior is also done.
Like many, my actions are largely based on experience rather than purely logic. While using QT, I did not quarantine one thing, plants. Ever since, we've had a snail issue. ..... They even managed to cross over from freshwater to saltwater! How ? It's bad enough they crossed a room to find another tank, but how did they manage to survive another salinity. Like the cockroaches that survived nuclear testing, nothing phases them. They have become an everyday reminder of QT.
So now everything that enters a tank, except my arm, has QT.