Betta splendent is a Labyrinth fish native to marshes across central and southern Asia. While selectively bred for flowing fins and bright colors, Bettas are adapted to live in pretty inhospitable environments compared to many freshwater fish. Their native habitat is weedy, plant-choked wetlands, with very little room to maneuver, and often stagnant water. They can thrive in these environments because their labyrinth organ allows them to breathe air, like gouramis. This allows them to live in water with a dissolved 02 at effectively zero, opening up areas uninhabitable by other fish. Due to the weedy nature of their natural habitat, Bettas are also adapted to live in very small spaces, which, when coupled with their ability to breathe air, makes them able to survive effectively in small bowls.
Like almost all fish, though, Bettas have gills, and buildup of metabolic and nitrogenous wastes in their bowl will kill them just like other fish, if at higher levels. It is therefore imperative to at least filter their aquarium/bowl, as well as keep it at an appropriate temperature, to let their metabolisms work the most efficiently, to promote health and reduce waste slightly.
Bettas often do fine in larger tanks, but owing to their fins, they are not very good swimmers, and can be sometimes bullied by rambunctious tank mates or out-competed for food.
Ideal tank size for 1 Betta and a couple of peaceful tank mates is 5–10 gallons with some floating or tall plants. They can also easily go down to 1 gallon or less (only if filtered however). There isn’t really a safe tank size for 2 adult male Bettas, as they will actively seek each other out and battle for dominance, sometimes to the death, but for a single male and a harem of 4–5 females, 10–15 gallons with no tankmates is ideal for watching their fascinating courtship and breeding behavior.