Generally speaking, quarantine is the way.
They exact answer is highly variable and is depending on exactly what you are trying to add. there is a whole group of forums on here based on treating and preventing diseases.
best advice I recall receiving when asked this question before is to start up your tank and cycle it, then add whatever corals/inverts you want, and THEN you start the fallow clock. No fish until at least 76 days after the last addition to the tank.
Adding ANYTHING from another tank restarts the fallow clock.
Once fallow, you must quarantine anything you add.
Corals and inverts you would QT for the fallow period, then good to move to the display if you don't have issues.
When you are ready to add fish, you must treat them each. Some fish are sensitive to meds/treatments in ways others are not, so really you must research each and every addition and tailor your approach. Some are known vectors for specific things like tangs and ich, so your research and approach must compensate. Something to keep in mind is if you are treating a tang with copper to stop ich, then beware what else you QT and treat it with, some other fish cannot handle copper well and will die.
