Betta fish are known for their beautiful fins, vibrant and varied coloring, and fierce attitudes. And since they are affordable and readily available in most pet stores, they're a great fish for an amateur aquarium enthusiast to begin a fishkeeping hobby. But although betta fish thrive best in solitary tanks, the popular choice of a small bowl is not the best way to keep one. Using a bowl over a proper aquarium will leave your fish bored and stressed, and likely shorten their lifespan.
Firstly, bettas are tropical fish. They will do best in warmer water, something that you could achieve in a tank of 5 gallons or more. But in a small bowl, even a tiny heater may heat the water too much, too quickly. All fish like stable temperatures, and this will quickly weaken your pet. Additionally, the small amount of water in a bowl means that it gets dirty quickly. A dirty tank means a fish more prone to diseases like fin rot and septicemia, and even some parasites. This goes doubly if you overfeed a fish!
If you plan to keep a betta fish, get a tank at least 5 gallons or more, and decorate it with a variety of plants. Fake ones are fine: bettas just like to hide in vegetation. Invest in a tank heater and thermometer, and keep the tank away from drafts and direct sunlight. Enjoy your pet, but make sure they get the best environment!