Have never kept ghost knife fish and it certainly sounds capable. I would not rule out the young Discus
. Until just recently I kept six Discus
and two were juveniles approx,four inches. (became too much like a job) The young ones ,would grab the threadfin rainbows that they were able to catch. I lost two of ten in this manner. I have since found new homes for the Discus
and currently have a small group of severums and another group of Angelfish. Had to move the threadfins to their own tank.
In a unrelated case,, I have caught many six to seven inch Largemouth bass on lures or surface plugs twice their length. Always wonder what their intentions were. Suspect they were reaction strikes and would not be surprised ,to observe this with Discus
,being the cichlids that they are.