I wish I'd known...
I give my 1 angelfish a juvenile endler a couple times per week. The first time, to make sure I wouldn't be torturing the food fish, I watched. The angel gulped the endler fry, spat it out looking unconscious, instantly gulped it again, end of story.
Since then, I haven't watched. I just notice when there aren't any food fish in the tank, then add 1-2.
But today I gave the angel 1 half-grown endler, instead of a fry. Never again. The angel sucked it into his mouth, spat it back out clearly injured, and it swam (vibrating) to the other side of the tank and back. The angel noticed, and did the same thing again. That second time, the endler came out alive, but too hurt to swim. A few seconds later, the angel actually swallowed it. The process lasted 2-3 minutes.
Lesson learned: Watch.
If you're feeding live animals to other animals, watch to make sure it's happening in a way that isn't causing prolonged or unnecessary suffering. For me, that means the angel will never again be given live food larger than a nibble, so the eating will be "gulp, gone" instead of prolonged or painful. The guidelines might be different in your tanks, for you to meet the same goal. But regardless, it's a topic to pay attention to. I wish I had been.