During the period that I had my Clown Knife fish, I found that he was very easily weened onto Shrimp pellets... He seemed to love these things! Another suggestion for a knife fish could be live foods, or as already mention freeze dried foods. Although most freeze dried foods do float which may be a problem when trying to get a mid to bottom swimmer to eat floating foods... Depending on the size (and species) of the Knife fish, you could try LIVE river shrimp. These work very well and are full of nutrients. Another thing which I fed mine was Super worms. These worms are often available at stores such as Petsmart, they're loaded with protein and aid in rapid fish growth.
Hope this helps.
Ryan