I have mine up and running all the time, and in line with other tanks.
If a new fish comes in, the tank is easily isolated by closing a valve.
A quarantine tank is not only there to isolate a new fish, it is there to get the new fish accustomed to your normal conditions, so I don't do anything different with the water unless treatment is called for. Some diseases become more virulent by higher temps, so whatever temp the fish is going to end up in, is what it gets in quarantine. .
The tank is also available to isolate fish that have been beaten up, or are a beater you want to separate.
I also use mine for starting new plants.

A pair of my Uruguayan pikes just paired up, doing slight damage to the 3rd, so in the pike went, and it was ready to go, as soon as the tank was needed.
