This site contains all about Siamese fighting fish in the wild.
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.