As others have said, they move quickly and breed... a lot. I quarantined and treated some feeder guppies, popped 5 in. The axolotls ate three in 24 hours. That was 2 months ago. Since then, the female has had two batches of fry, none of which are dying or getting munched on. I am rapidly worrying that I'll have to start managing population control, myself, since the axolotls aren't doing it for me.
I've also been breeding cherry shrimp, like David F., but I haven't gotten around to popping a few adults in the axolotl tank, yet, so I can't share experiences on that front. I will say that RCS are insanely easy to breed.