No need to add more bacteria for now atleast.
Take the fish out. Test your levels. Test phosphates next - your dry rock might be leaching this and at a higher level it’s harmful for fish and corals.
I don't think so. I used Reef Saver rock which has no dead material on it at all.
Are you using rodi water for your salt and top off?
Yes, I have my own RODI water-maker.
Almost positive they died from disease, etc. I don't think there is an issue with your tank.
That's what I would think too, but they seemed fine at the LFS. That's why I'm wondering if I should do a large water change if so?