does the knife have hiding spots and places that shield him from the lighting? if hes just out there under the lighting he might feel stressed and panicky, create a couple options of hiding spots for the fish and see if it relaxes and settles in more. Also, these fish hunt in extremely dark waters and are generally nocturnal feeders until they get comfortable in the tank, you might want to try dropping some suitable sized feeders in the tank overnight and check the results in the morning (generally small comets are slow enough to be an easy meal). I've never seen a knife, even a dying knife, turn down a feeder. What other types of fish are in the tank? If there are other aggressive heavy eaters the knife also may not have the option of getting a good pick at the food. They are reclusive and ambush eaters at that size generally, so take that into consideration. Good luck and keep us posted!