All the Betta species are small fishes, but they vary considerably in size, ranging from under 2.5 cm (1 inch) total length in B. chanoides to 12.5 cm (5 inches) in the Akar betta (B. akarensis).
Bettas are anabantoids, which means they can breathe atmospheric air thanks to a unique organ called the labyrinth. This accounts for their ability to thrive in low-oxygen water conditions that would kill most other fish, such as rice paddies, slow-moving streams, drainage ditches, and large puddles.
The various bettas can be divided into two groups, based on their spawning behaviour: some build bubble nests, like B. splendens, while others are mouthbrooders, like B. picta. The mouthbrooding species are sometimes called “pseudo bettas”, and are sometimes speculated to have evolved from the nest-builders in an adaptation to their fast-moving stream habitats.
There is often much confusion in terminology regarding these fish. Siamese fighting fish, B. splendens, are frequently sold in the United States simply as bettas. Fish fanciers are thus often unaware that, as of 2006, there are around 65 species classified within the genus Betta. A further source of confusion is that while the generic name Betta is italicized and capitalized, when used as a common name it is usually not capitalized. The common name of Betta pugnax, for example, is thus Penang betta.
Siamese fighting fish, B. splendens, is often referred to as betta in the U.S., leading to some confusion
The name Betta (or betta) is pronounced /?b?t?/. That is, the first part is the same as the English word bet. By confusion with the name of the Greek letter beta, the name is often pronounced /?be?t?/ in American English, and may be misspelled with one t. The name of the genus is unrelated to that of the Greek letter, being derived from the Malay word ikan betah (“persistent fish”).