Behavior
Male and female Bettas 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. Females and males will display horizontal bars (unless they are too light a colour for this to show) if stressed or frightened. 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). Bettas sometimes require a place to hide, even in the absence of threats. Bettas 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, though individual females demonstrate a wide range in level of aggression. The aggression of bettas has been studied by ethologists and comparative psychologists. Bettas 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. Bettas, like other fish, 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 the plant leafs reach the surface, they are useful for males to base their bubblenests on.

Bettas with other species
Because of the aggressive nature of the Betta splendens species, breeders and owners take consideration of other species inhabiting the aquarium:

Two or more Males: Male betta splendens do not fight to the death in the wild; when one fish has won the fight, the loser retreats to safety. In an aquarium, however, there is no retreat, so the victor fish continues attacking the loser, often resulting in the loser's death; therefore, hobbyists rarely house two male B. splendens in the same tank unless they are (a) separated by a partition; or(b) they are from the same batch of eggs and are immature; A Male and a Female: In the wild, females stay clear of males, except during mating. When cohabiting in tanks, males might kill females, and are generally kept apart unless (a) they are juvenile siblings, (b) they are breeding,or (c) there is a partition.Often, before breeding, breeders use a partitioned container to allow female display without risking harm by the male.

Compatible fish of other species: Hobbyists put Betta splendens in tanks with other species after careful research and preparation. Common tankmates include platies (moons), Corydoras catfish, and loaches. Females can share a tank with danios and tetras, but males should not be kept with these species as they tend to nip at their long fins. Shrimp are popular tank-mates because, provided with sufficient natural plant cover, they keep the tank clean without causing stress to the bettas.

The success of a betta in a community aquarium, however, is largely dependent on the particular betta's level of aggression. Whereas some bettas make wonderful community fish, particularly belligerent or skittish bettas are best housed alone. Bettas are very aggressive towards long-finned fish, the guppy for example.