Hey guys, I am trying to figure out what killed my new six line wrasse that I got just 4 short days ago. It was less than one inch long and was getting along fine with the banggai cardinal (~2 inches) and juvenile clown (~1 inch) that I have in my 10 gallon tank. I came home today and found it dead in the sand with a huge gash in its neck. I have attached a few pictures.
What do you think did this? I have absolutely no idea. My strongest suspicion is the banggai, but it didn't show any signs of aggression towards it before. I also don't know what a fish bite looks like. This gash seems pretty clean cut, right along the gills and jaw line, maybe too clean for a banggai bite?
Other possible suspects (I really have no idea):
Bubble tip anemone?
Relatively large hermit crab?
I fed the tank some live brine shrimp the night before the murder. Could those have eaten their way out? Seems very unlikely...
Maybe it nibbled some dried IC-Gel that I used to glue some corals down that messed with its insides?
Help me solve this mystery. I am sad. Thank you.

