Help with New additions dying

[Aquattro]

Coraline growth isn't really indicative of anything, really. I get hardly any. You also stated in your other thread that all nutrients are 0, but chaeto grows really well. That means there's something there to feed it.
By not doing regular water changes, you can create an environment where the water is not nearly optimal, but existing corals adapt. New ones, not so much.
But, if you've had this with both, probably not the cause in this case.