Betta Tanks: Small Does Not Mean Bowl
Bettas are sold as low-maintenance and are routinely kept in conditions that are not.
Space
A single betta wants a minimum of around 20 litres with a lid. They are jumpers, and an open bowl invites accidents.
Still water and cover
Bettas come from slow water. A filter with an adjustable or baffled outlet, plus planting to rest near the surface, matters more than volume.
Heat
They are tropical. In most of India an unheated room is fine for much of the year and not in winter — a heater and a thermometer settle it.