U want big teath and a fish that swims alot. You could get a pacu, a gar, Red bellies, African tiger fish, mbu puffer, snakehead, or maybe a shark,(JK) but in a 55 your limited to killers, even cichlids with large teeth will outgrow your tank. Jack dempseys have teeth but dont swim much. They are many false gars that have teeth that stay small but they are boring. Look on here for monsters for a 55