It really depends on how you define carnivore. Most fish will eat smaller fish if they are much larger. If you are looking for a strict predator, as Terrance mentioned, you are pretty limited. The only one I can think of off the top of my head would be a grass pickerel or redfin pickerel. I have a few of them and keep them in 29g tanks without an issue, they spend 95% of their time just hovering somewhere so I don't consider them as needing large swim space.