How do I know which of my fish is carnivorous and what can I do about them eating my other fish?

Both could have eaten it. Most fish are omnivores and eat both plants and meat. Shrimp and bloodworms are on the menu, as well as smaller fish. Your glowfish could have been killed by either or them or have just died of natural causes. Fish tend to "clean up" pretty fast. By clean up, I mean dispose of the body. ;) Well yeah, you can keep feeding them what you're feeding them, but every once in a while another fish might go missing. It's a fact of life. Sorry I can't help you find out which one ate it, that's a pretty hard thing to find out anyway. Hope this helped! :)