Need some Help

[Frogger]

I have heard stories of mantis shrimps coming in the small crevices of the rocks. Only to grow and become fish eaters in the future. They can be really good hiders in the tank. I realize you started with dead rock, what about the corals that you have added to you tank were any of them attached to rock?

Maybe the rock shifted and the fish got trapped.

Watch the tank late at night with a blue light and see if you see anything.

If none of your other fish are showing signs of stress then maybe it was just a coincidence, like I said before fish die, but usually their behaviour changes before they die.