When Do Bettas Start Showing Aggression?

Betta fish, also called Siamese fighting fish, are prized by aquarium hobbyists for their brilliant colors and large, flowing fins. You can keep a single male in a tank as small as 1 gallon or, under the right circumstances, in a community tank. Although peaceful around fish of other species, they are notoriously aggressive around their own kind.

  1. Male to Male Aggression

    • Male bettas become aggressive toward other male bettas on sight, and they immediately begin attacking each other by ripping at their opponent's fins. The fight is to the death, and wounds often result in the deaths of both fish. Because of this natural aggression, male bettas should be kept in their own tanks or in community tanks in which they are the only male betta. Female bettas are relatively peaceful fish, and you can keep several in the same tank.

    Male to Female Aggression

    • Upon sight of a female betta, a male betta's thoughts immediately turn to raising a family. He darts around the female, spreading his fins and undulating his body. If the female is reluctant, he will attack her, often killing her. For this reason, males and females should be introduced gradually, either by placing one of the fish in a jar inside the tank or by dividing the tank with a sheet of glass or plastic. This gives the female a few days to prepare for the encounter. After spawning, the male becomes aggressive toward the female, who is in a weakened condition. She should be removed from the tank to allow the male to raise the young on his own. A tank containing a male and female betta should have plenty of hiding places for the female.

    Fry

    • Once the baby fish, called fry, hatch from their eggs and begin swimming horizontally, you can remove the male betta from the tank. Don't leave him there after the fry become independent, because he may begin thinking of them as food. The males begin to show signs of aggression toward each other at about eight weeks of age, and they should be removed to separate tanks at this time.

    In the Community Tank

    • In a community tank where he is the only one of his species, a male betta makes a good tank mate for other peaceful, slow-moving fish. Because of his long, flowing fins, he is likely to become a victim to fin-nippers; the damage can lead to infection and death. If you notice another fish actively chasing a male betta around the tank or if you see fin damage, remove the betta to a separate container.