Something is killing my corals and has killed all my molluscs

[PeterLL]

More details here: https://www.ultimatereef.net/index.php?threads/Did-Zoas-nuke-my-tank?.852308/&share_type=t

In short, something has killed all my molluscs, my zoas, mushrooms and leather coral. They are all dying and covered in some weird white substance.
The tank is fallow and nothing has been touched in about 2-3 weeks before this happened.
Shrimp, bristleworms, macro algae’s are all fine, even the gorgonian.
Parma’s are fine (0/0/2)

Then a picture about 5 weeks ago for reference:

Thank you so much for any help

 

[MnFish1]

Sometimes when leather coral dies - it releases substances that are toxic to other corals (same with Zoas - I think) - do you run carbon? Did anything else change in your tank? If you are sure things are dead - especially soft corals - remove them ASAP... Are you other parameters normal?

 

[vetteguy53081]

Lights seem quite bright. Run whites at 18% and blues at 75-80%. Additionally how much water flow are you providing and what type of water are you using ( tap, well, RO ) ?