Peeps gotta stop arguing the "it's not natural to stick a little fish in with a big fish in a glass box for the big fish to eat" point because if you're gonna say that then it makes you appear slightly idiotic (no offense to anyone) because:
1. You yourself are keeping a large predatory fish (I assume) in a glass box which is hardly an exact replica of the Amazonian River Basin or whatever body of water your fish came from.
2. I suppose that fish eat prepared pellets packed with all of the essential vitamins, some spirulina, and whatever else goes into the "complete" pellet, in the wild?
Also, in my experience, the fish I've kept seem to grow fastest/best when fed mainly live food. Granted, I've stopped doing that due to the expenses of feeding live 24/7, it doesn't change the fact that the best growth rates I've ever seen were on the live diet. Not to mention the best way to condition fish for breeding is with live food, IME.
Also, predatory fish seem to never grow to their full potential in the home aquariums, despite all of the technological advancements in the fish keeping field, the "superior" scientifically formulated pellets, a controlled environment, fresh uncontaminated water with every water change. Yeah, the size of the environment plays a large part too, but the diet is also just as important IMO.
But yeah, anyways, feeding a mouse to an oscar is just wrong.