I'm so sorry to hear about your fish! I had something really similar happen to me, all the fish just suddenly died on me with all inverts and corals being fine. Never did figure out what happened for sure, but I suspect a bloom of bad bacteria? I added some filter media from an established tank and a bunch of pods, which helped, and I haven't had any fish loss since. Just mentioning in case it helps.
Anyway, I really hope things improve from here!
