You know exactly what you doing. This is not an issue of knowledge or skill, you have both. It's not an equipment issue either. I did not ask how you manage ALK/CA. A calcium reactor? Kalk? I think what I would do at this point is change salt. I keep coming back to instant ocean for some reason having tried many. Here is a thought! Many of us have a HUGE water container to store mixed water for changes; is it outside? Could it have become contaminated?
Is it an issue of feeding more? I feed my fish a lot and it might help the coral as well.
Is it an issue of feeding more? I feed my fish a lot and it might help the coral as well.

