Are there any symptoms before a fish passes? Mine for instance hid and didnt eat and then wham they were dead within hours of the first sign.
I gotta be honest, I still blame myself for my 125g nightmare. Even though I tried my best I still feel I did or didn't do something. BUT I kept things rolling. I still worry about anything everything I add. I wouldn't let it get you too down, it happens. And I know how you feel exactly, is probably the best thing I can say.
You can move past this though, so don't let it be the end of your hobby. You've sunk all this cash into it and you enjoy it so keep going. What about the qt tank, anything funny going on in there since all your fish spend some time in there?
Good luck with your fish, I hope they make it. But if not, you know we're here for you.