Sorry to hear about your Ray. I hope you can figure out the problem before much more damage is done. I don't have much input on what it could be, but I doubt it is the Gar.
My Bichir is only 9" but man he will eat ANYTHING he thinks he can put in his mouth. I've seen him maul fish like a terrier with a steak, and he sneaks up on them too!
i would say ornate mine just took a chunk out of my aba aba knife and they were almost the same size now aba is like 2 inches shorter lol. I have 1 vittatus tiger fish 1 hepsetus ode tiger pike 1 aba aba knife 1 endi 1 delhezi 1 albino sengalus and the 1 ornate and the ornate is the only agressive fish in the tank.
I highly doubt it is one specific homicidal fish. Because If this killer was aiming at killing fish naturaly the smaller ones would be the target, no? Could the Indos have killed the thin-bar? Cause I mean honestly if a fish wanted to go killing everything i would think the severums would be the first ones down. Flowerhorns are tough not sure what took them out.
But after looking at your stock I am thinking...
Fire eels or Aro's IMO. Not so sure about the Fire eels though i'm just zoning in on the big guys.