Males and females flare or puff out their gill covers (opercula) to appear more impressive, either to intimidate other rivals or as an act of courtship.
Other reasons for flaring can include when they are intimidated by movement or change of scene in their environments.
Both sexes display pale horizontal bars if stressed or frightened.
However, such colour changes, common in females of any age, are rare in mature males due to their intensity of colour.
Females often flare 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).
Betta splendens enjoy a decorated tank, being a territorial fish it is necessary to establish territory even when housed alone.
They may set up a territory centered on a plant or rocky alcove, sometimes becoming highly possessive of it and aggressive toward trespassing rivals.
This is the reason why when kept with other fish the minimum tank size should be 45 litres (about 10 gallons).
Contrary to popular belief, bettas are compatible with many other species of aquarium fish.
Given the proper parameters bettas will be known to only be aggressive towards smaller and slower fish than themselves such as guppies.

The aggression of this fish has been studied by ethologists and comparative psychologists.
These fish have historically been the objects of gambling; two male fish are pitted against each other to fight and bets are placed on which one will win.
One fish will arise the victor, the fight continuing until one participant is submissive.
These competitions can result in the death of either one or both fish depending on the seriousness of their injuries.
To avoid fights over territory, male Siamese fighting fish are best isolated from one another.
Males will occasionally even respond aggressively to their own reflections in a mirror.
Though this is obviously safer than exposing the fish to another male, prolonged sight of their reflection may lead to stress in some individuals.
Not all Siamese fighting fish respond negatively to other males, especially when the tank is large enough for each fish to create their own designated territory.
Males and females flare or puff out their gill covers (opercula) to appear more impressive, either to intimidate other rivals or as an act of courtship. Other reasons for flaring can include when they are intimidated by movement or change of scene in their environments. Both sexes display pale horizontal bars if stressed or frightened. However, such colour changes, common in females of any age, are rare in mature males due to their intensity of colour. Females often flare 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). Betta splendens enjoy a decorated tank, being a territorial fish it is necessary to establish territory even when housed alone. They may set up a territory centered on a plant or rocky alcove, sometimes becoming highly possessive of it and aggressive toward trespassing rivals. This is the reason why when kept with other fish the minimum tank size should be 45 litres (about 10 gallons). Contrary to popular belief, bettas are compatible with many other species of aquarium fish. Given the proper parameters bettas will be known to only be aggressive towards smaller and slower fish than themselves such as guppies.
The aggression of this fish has been studied by ethologists and comparative psychologists. These fish have historically been the objects of gambling; two male fish are pitted against each other to fight and bets are placed on which one will win. One fish will arise the victor, the fight continuing until one participant is submissive. These competitions can result in the death of either one or both fish depending on the seriousness of their injuries. To avoid fights over territory, male Siamese fighting fish are best isolated from one another. Males will occasionally even respond aggressively to their own reflections in a mirror. Though this is obviously safer than exposing the fish to another male, prolonged sight of their reflection may lead to stress in some individuals. Not all Siamese fighting fish respond negatively to other males, especially when the tank is large enough for each fish to create their own designated territory.
Siamese fighting fish