Well, both of my bichirs are very active and are forever chasing down an array of food in their tanks (quite vicious lil' things actually, I've never seen more carnage hah). And they're successful almost all the time.
But the knife fish I've seen (never owned one) seemed very disinterested in hunting and didn't get stellar reviews from either owners or LFS for hunting down food well. Maybe they weren't housed properly or just happened to be disinterested individuals. *shrugs*