Looking at your parameters (I would get your LFS to double check them) I don't know what would killed all the fish.
Were the rocks in bins?
When did you test your water parameters?
I'm wondering if it was something in the rock, but the inverts are usually the ones to go first....something is amiss....
Ok, reading your first post and then the last one, what fish did you have in there, unless I am crazy the two posts don't match...?