Boxfish

From Freepedia

The Boxfish are a group of squared, bony fish. They come in a variety of different colors, but are a very peculiar looking fish. They swim in a rowing manner. They are good community tank members and can be as small as the size of a die. They are not thought of as an ordinary aquarium tank mate, but are quickly gaining popularity. They do pose a hazard in the community tank however. They are capable of releasing a very powerful toxin which can kill other fish on contact. They generally only use it when threatened, but can become disturbed easily with aggressive tank mates or overcrowded aquarium.