Goldfish
In a fish tank, fish will eat nearly anything. So if you have 30 fish, the fish will start eating any fish that looks sick or different. This will continue until you end up with one very large fish, usually the goldfish. Much like when a message board gets run over with a few friends who want to monopolize it. They will ceaselessly troll until anyone they aren't friends with leave. Then they'll move on to another site and do the same until it too ends up with one or two users.
That messageboard used to be a good place to post until those goldfish came and destroyed it.