eating fish

I picked up a mandarin last Sunday. On Tuesday I noticed some notches in its tail. On Wednesday it was worse, so I started searching for ideas. I came across a single thread that mentioned someone seeing a bristle worm eating their mandarin's tail fin and the mandarin just kind of chillin...

I have red and blue leg hermits and just this morning, I saw my hermit crabs eating my Anthias. All was left of the Anthias is it’s bones. When they sleep in my tank they go in the rockworks and so does my Chromises (which are 5 months older). The Anthias and the rest arent sick or anything