I think it is cruel to the fish because the duckling is clearly far too big for it to swallow. It was having difficulty and could have possibly choked, or worse. I feel this way about any large live animal being used as feeders. What if a large rat or in this case duckling cut through the throat of the fish with it's sharp claws? In the wild this does happen, but that is natures way. A large tank in someone's living room is not the wild, and therefore, a forced man made circumstance that is putting the fish at risk. We have a responsibility the second we move an animal out of the wild, to care for it in a safe and humane way. Plus, there are plenty of healthy alternatives to live animals that would be more beneficial to the fish.