Behavior

Males and females flare or puff out their gill covers (opercula) in order to appear more impressive, either to intimidate other rivals or as an act of courtship. Other reasons for flaring their gills is that they are startled by movement or change of scene in their environment. Both sexes will display horizontal bars (unless they are too light a color for this to show) if stressed or frightened; however, such a color change, common in females of any age, is very rare in mature males. Females often flare their gills at other females, especially when setting up a pecking order. Flirting fish behave similarly, with vertical instead of horizontal stripes indicating a willingness and readiness to breed (females only). In fact king Bettas flare their fins and gills as a sign of aggression or flirting with other fish. Bettas sometimes require a place to hide, even in the absence of threats. They may set up a territory centered on a plant or rocky alcove, sometimes becoming highly possessive of it and aggressive toward trespassing rivals.

On average, males are more aggressive. The aggression of this fish has been studied by ethologists and comparative psychologists. Siamese fighting fish will even respond aggressively to their own reflections in a mirror; use of a mirror avoids the risk of physical damage inherent in actual conflict, although it can lead to stress in some individuals. Like other fish, the fighter may respond to the presence of humans and become trained to respond to feeding cues (such as a hand placed over the water's surface). They are quite curious and will watch humans going about their business nearby. When plant leaves reach the surface, they are useful for males to base their bubble nests on. Ethical issues are still debated with the growing activity of pitting males together. Audience bet on the fishes while the bettas savagely attack each other. It is advised to isolate males from one another to avoid such aggression.