What makes an aquarium 'safe' to add 'difficult' inhabitants

[MnFish1]

So - I keep hearing one needs to do x, y, z before adding (insert difficult to keep coral or fish here). I wanted to see what everyone thinks is the MOST important thing. I've heard every one of the choices below - so I'm interested in what everyone thinks. If you have another factor that you think is more important - add it in the comments. Thanks.

 

[Retro Reefer]

IMO abundant coralline growth is one of the best indicators tank is maturing and ready for harder to keep corals.