I've lost a blue tang, four false percs and a peppermint shrimp with no good explanation.  The other day I happened to glance at the base of a rock and saw some nasty looking worm-like thing protruding from a hole.  It had to be a good 4 inches long.  I didn't see a head, but it looked like a segment of a bristleworm.  I tried to pin it with my fish feeder, but it retracted back into the rock.  Haven't seen it since.  I'm wondering if this may be the culprit that's been killing off my fish ...