Thoughts on unnexplained (and very sad) die-off

[howdy20012002]

I don't think it is a disease personally.
I don't know of any disease or parasite that can kill both fish and inverts.
IME typically inverts are fairly resilent to disease.
I think it must be something with the conditions of the tank
perhaps a contamination?
are your corals doing okay? what kind of corals do you have?
did you add any rock recently and did you test for copper if you did.
LR can hold copper and it will leech slowly back into your system.
I would definitely add carbon to see if you can contain any contaminents.
although the copper wouldn't affect the fish, it would definitely kill your inverts..and alot of other things such as bristle worms, amipods etc...
do you still see the critters swimming around your tank when the lights are off? see bristle worms when you feed? or any snails or crabs still alive?
if not, then I would definitely suspect copper...(although if your corals are doing fine..probably not copper either)
this die off could certainly cause the conditions of your tank to go south..which in turn would make your fish more prone to disease that they had previously been able to fight off.
however, diseases aren't something that linger around in your tank without being noticed...most diseases will either become noticeable or not.
so if you haven't noticed any disease symptoms within the last 3 months, not really sure what could be killing beyond something in your tank.