Betta fish, also known as Siamese Fighting Fish, are known for male against male aggression. Although individual behavior may vary, most male betta fish in the pet fish trade need to be kept in separate tanks, and shouldn't even be able to see other males. Fighting in betta fish can be potentially lethal, so owners need to take effective steps to prevent their fish from fighting.
What Is Fighting in Betta Fish?
The most common fighting behavior in betta fish is known as "flaring." In this demonstration, a betta fish will push both operculums forward to suggest a larger body size. This is similar to a puffer fish expanding and flaring its spines when under duress.
Other behaviors involve physical interaction between two fish. Physical interactions may include one fish ramming, or swimming directing at and/or into, the other fish, or biting or nipping at the fins.
Why Do Betta Fish Fight?
Betta fish fight to establish a territory, including food resources, shelter, and access to females. This is a common cause of aggressive behavior in many different fish species.
There is much debate over whether this fighting behavior is innate or a consequence of how betta fish are reared. Studies have shown that bettas reared in a group have less aggressive tendencies. It is hard to know with some suppliers how your betta fish were reared and how aggressive they may be, and may take a few weeks for you to be able to tell their level of aggression.
Betta fish have a history of being kept as competitive fighters. Studies observing competitive fights have shown that fish raised in isolation, without other betta fish, tend to be more aggressive and fight for a longer period of time. Just like dog fighting, keeping fish for this specific function is a severe welfare concern.
Although less common, female betta fish can be aggressive in tanks that are overstocked. Female bettas are commonly kept in a small group, known as a "harem," and individual fish may be more or less aggressive than others in a group, often leading to an established hierarchy. Once established, the addition of new females to the harem may result in increased fighting as a new hierarchy is established.
Betta fish may be aggressive to fish of other species kept in the same tank. It is critical to add fish that are not aggressive and good community players to your aquarium or tank. It will depend on your betta's individual personality to determine if they can be kept with additional species. Some bettas are too aggressive to be kept with any other fish. It is best that betta fish are added to the tank last in order to decrease any potential aggressive interactions.
Signs of Fighting in Betta Fish
The most common sign of fighting is active fighting, including ramming or nipping. If you do not see your fish actively fighting, you may see other clinical signs, including missing scales, torn fins, or increased hiding. Severe signs include lethargy, decreased appetite, prolonged hiding periods, and sudden death.
"Fin rot" or fraying fins is a common problem in betta fish. Often, this is a general sign of illness and a poorly functioning immune system. Another common cause is overdecorating with lots of items that can tear betta fins. Fighting will often result in other clinical signs, not just fin damage.
How to Stop Fighting
The best way to stop your fish from fighting is to only have one male fish per tank. Provide a visual barrier between the two tanks so the fish cannot see each other at any time. Simple tank backgrounds or a piece of cardboard are effective visual barriers.
Visual toys including mirrors or mirrors placed near the tank should also be removed. Bettas have been known to aggressively respond to their own reflections. Although these may be considered "enrichment" items, these items are known stressors in bettas and should not be added to betta tanks. Injuries can occur when betta fish attack toys or the sides of their tank.
Treatments have been attempted with marijuana and Prozac to decrease aggressive betta fish behavior. Bettas responded with decreased aggressive behavior with both, but became tolerant of the marijuana dose. Bettas also responded with a decrease in aggressive behavior with Prozac, but repeat treatments have not be studied. Always consult your veterinarian before embarking on any treatments for your pet fish.
Iwata E, Masamoto K, Kuga H, Ogino M. Timing of isolation from an enriched environment determines the level of aggressive behavior and sexual maturity in Siamese fighting fish (Betta splendens). BMC Zoology. 2021;6(1):15.
Ichihashi T, Ichikawa Y, Matsushima T. A non-social and isolate rearing condition induces an irreversible shift toward continued fights in the male fighting fish(Betta splendens). jzoo. 2004;21(7):723-729.
González SC, Matsudo VKR, Carlini EA. Effects of marihuana compounds on the fighting behavior of siamese fighting fish(Betta splendens). PHA. 1971;6(3):186-190.
Lynn SE, Egar JM, Walker BG, Sperry TS, Ramenofsky M. Fish on Prozac: a simple, noninvasive physiology laboratory investigating the mechanisms of aggressive behavior in Betta splendens. Advances in Physiology Education. 2007;31(4):358-363.