Betta Splendens are commonly known as Siamese fighting fish. This is because the males of this species cannot be kept together or they will fight each other, sometimes to the death. Sometimes you will see a single male betta flaring his gills at his reflection in the aquarium glass because he thinks that his reflection is another male. Male bettas flare their gills in order to appear larger and more threatening.
Bettas are originally from southeast Asia. They are from the family Belontiidae. All fish from this family possess a labyrinth organ that allows them to breath atmospheric oxygen. In fact, they must have access to oxygen from the air or they won't survive. Bettas also have gills that allow them to take in some oxygen dissolved in the water, but this doesn't provide enough oxygen for them to survive. You will frequently see bettas come to the surface for a gulp of air.
Bettas in the wild don't have the brilliant colors or the extravagant fins and tails that you see in pet stores. Wild bettas are reddish-brown with a little bit of red, green, or blue on them. The colorful bettas that you see in pet stores today are the result of selective breeding of betta offspring to possess the brilliant colors and enlarged fins. Through selective breeding male bettas now come in red, blue, purple, and green. For more on breeding bettas please see the article Breeding Betta splendens, which also contains a short video of two bettas spawning.
Female bettas, while colorful, don't possess the brilliant colors of the male or the enlarged fins. However, they are still beautiful fish, and you'll need a female betta in order to have a breeding pair. It has been my experience that female bettas are more difficult to find in pet stores than the males.
In pet stores you will generally find male bettas for sale in small fishbowls. Because they can breathe atmospheric oxygen they live just fine in these bowls, however, I do think that they would prefer some more swimming room and so moving them into an aquarium is best. They can be kept in community tanks, where no other male bettas are present, and with peaceful fish that won't nip their long flowing fins.
You must use caution when keeping female bettas together with a male. I once had a male betta kill a female that was placed in the tank with him. If you do keep them together it is best to keep at least 2 females to 1 male and to provide plenty of hiding places for the females (plants and decorations). In general, it is usually best not to have female bettas in the same tank as the male unless they are spawning. You can keep the females together in the same tank without the worry that they will fight one another.
Diet and Water Conditions
Bettas readily eat dried food, such as dried tropical flakes, tubifex worms, bloodworms, and small crustaceans, such as brine shrimp. They also like frozen food (thaw it first). Some companies now sell food that is specially formulated for Bettas.
Bettas do best at a neutral pH (pH of 7.0) and the water temperature should be between 77-83 degrees Fahrenheit or about 25-28 degrees Celsius. When I see Bettas kept in unheated fishbowls in pet stores I often wonder if the water temperature is too cold for the health of the Bettas. Because most people keep their air temperature below 77 degrees a heated aquarium will be best for the health of your betta.