Ouch .. sorry to hear.
Generally speaking to me a tank crash is something you can indeed track (ie. NO3, or NO2, or even NH4 in extreme cases). Or there's an off smell to it, or some such. It's usually corals that show distress first. Fish showing first .. arrrgh, I don't really know. But I wonder if it could be that they were both new acquisitions and didn't transition well (and it's just coincidence on the timing), or maybe something more sinister like disease or something ??
I'm so sorry for your losses.