Tiny little pistol shrimp are indeed a thing. Either juveniles of larger species, or adults of tiny species.
When you catch whatever it is, if it turns out to be a pest, you can use clove oil to euthanize it humanely. You should have clove oil on hand anyway, for when tank inhabitants are sick or injured enough to need euthanasia.
The critter may not have killed your fish. Hermit crabs and other scavengers will make an intact dead fish look predated upon pretty quickly.
When you catch whatever it is, if it turns out to be a pest, you can use clove oil to euthanize it humanely. You should have clove oil on hand anyway, for when tank inhabitants are sick or injured enough to need euthanasia.
The critter may not have killed your fish. Hermit crabs and other scavengers will make an intact dead fish look predated upon pretty quickly.