Bottoming out nitrates wouldn't likely cause this issue unless you went from a very, very high ppm number to zero relatively quickly. If the bottoming out happened gradually, you would likely see coral start losing color and becoming very pale over time before any tissue necrosis started. I'd likely look elsewhere first.
After testing water parameters (locally with test kits), check all your magnets and any pumps with impellers. Look for rust and damage. I have seen impellers get damaged and once they do, it can start to rust or grind the impeller. VorTech wetsides should be checked if you have any. Probe magnets are often to blame. Checking this should be higher than #8 in my opinion - it should be really
mystery #3 when diagnosing weird coral deaths -
@derek_SR is secretly gatekeeping this one.