My answer then, I guess, would be the very finicky, slow growing Acropora. The ones that peel if you look at them wrong. There is a visible stage of a tank where coralline is actively growing. Detritus is being broken down very quickly. And corals are growing at a rapid rate. This is the stage where I feel confident putting in the most sensitive corals. I'm not sure what the specific factors get us to that stage. But it is more a visual observation for me, than something I can test for.