But Nature isn't captivity.
In Nature, the prey has a chance of escaping and going on to live a normal life. In captivity, the animal is basically thrown into a tank to be killed within a short space of time. It has no place to hide and no chance of escape. I have often seen half-dead goldfish, crabs with missing legs, and so on... so the idea the death is "quick and painless" doesn't even apply. I remember seeing a goldfish with its back half bitten off but the head floating and obviously alive and the gills breathing. Even in lesser cases than this, the feeder fish must spend hours or days being terrified by being in a tiny space with a much larger and plainly dangerous animal.
I guess it depends on how sympathetic you are to other animals. If you don't mind seeing pain and suffering, then fine, feed crabs and goldfish to whatever. But if you're the sort of person who does care about animals, then there's no way to justify what is essentially cruel and unnecessary. I keep fish, and became a zoologist, because I like animals; for me, that's the end of the story.
Cheers,
Neale
but isnt that just apart of nature? do you think that puffers never eat a crab or two in the wild?