Actually I'm talking about loachs not knifes.A p. niger would never be able to eat a clownknife. Their mouths are like carp's. They prefer to eat pellets, worms, detritus, plant material, and even fruit. They are gentle giants and are non aggressive. The like to sift through sand and gravel for food. Be aware if you have a nice planted tank that they will uproot plants and other decor in your tank.
My tank is quite bare (no plants) with a large piece of driftwood and natural riverrock substrate. My pacu. barbs, bocourti etc would take out most plants.KNH