OK basics time
How do you source your water, do you make your own or LFS?
Have you tested the water before you add to the tank ?
If so what are the readings before you add Salt any reside if phosphate or minerals TDS wise ?
I am not an expert amd don’t confess to be,. However from the sound and look. I suspect you have had a KH swing your PH is a little low as well . Corals sometimes crash months after a change. Whilst your Calc and Mg are on the high side I wonder if this has had a negative impact on your KH at some point. Do you run any carbon reactor GFO etc does your skimmer pull waste through and now often as mentioned do you water change and what %
Nopox is fine but it works in the reddield ratio it needs phosphate to work ( like most carbon dosing) If you could point us in the right direction regarding the above questions I am sure someone will be able to advise further
I hope you get to the bottom of this , heart breaking to have a fantastic tank amd in the blink of an eye it goes down hill for no apparent reason