Fish Size. An old aquarium adage states "if a fish can fit into another fish's mouth, chances are it will end up there." Most fish are opportunistic when it comes to food, and even relatively peaceful fish will try to eat other fish if they think they can.

Why is my fish eating the other fish?

New Fish in the Tank

Just like any other species of animals, in fish, territories are marked. Therefore, the existing fish tend to keenly observe the new fish and the space they occupy. If the fish feels threatened in some way, they will come up with a plot to eat up the new fish.

What fish will kill other fish?