thats what i used to think till i saw one rip an african cichlid in half, eat the head, and let the rest float away
i think they are more "opportunity feeders"
That's pretty cool that those plants sprouted from seeds. I'll wager that there's a good chance that they are from the tomatoes. A large percentage of tomatoes are grown hydroponically so if a species of tomato can grow healthy with out soil I would imagine it would not be too much of a stretch to say that a seed could germinate and sprout fully submerged in an aquarium. I say take half of them and plant them in some soil and put the other half in that 55 you have cycling.
Just so you know the leaves of tomato plants are considered poisonous. If you feed them to your dog/cat/lizard they will most likely get sick. Do not know if it is the same for fish.