That sucks. I understand how you feel.
I had all 10 or so of my big ropefish die within 3 days of some apparent disease a few years ago. Just one after the other got weird discolored spots, started swimming weird and died. They were with other fish who remained absolutely fine, including my marble cat who I still have, so obviously it was something that only effected that species or family of fish. Never did figure out exactly what it was.
Now I'm scared to get more lest it happen again.
Did you forget to add dechlor? Or maybe the power went off overnight and they didn't have enough oxygen? The only time I woke up to dead fish like that was when the power got turned off to one of my SW tanks and everyone but my moray and grouper was dead.