Wow, there's a lot of conflicting information on this thread.
I have kept fish tanks for 20 years, and I currently have 7 tanks up and running. Everything from a 5 gallon tank with a betta, to a 60 gallon tank with cichlids. Soon to be 8, as I'm upgrading the 60 gallon and moving some fishies around

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Anyway: Bettas can live in a 1 gallon bowl, but it is not the best environment for them. I mean you can live in a 6X6 foot house too, but do you want to?
Bettas do need heated water (78-82 degrees seems to be best). They need hiding places, and they need room to move around. You take your inactive betta from a 1 gallon or smaller tank and place them in a properly heated tank and it's an amazing transformation. They're active, and love to swim and play. I have a betta living in my 26 gallon community tank, and he is honestly the most active fish in there.
The other reason to keep them in a larger tank is ease of care. Anything lower than 5 gallons is much harder to keep cycled and clean. You should definitely clean the tank more than 'when it looks cloudy'. Instead, change 25% of the water once a week. This will keep the tank cycled, (I'll explain cycling below) but keep fresh water. Remember, the water is your fish friend's entire environment. Would YOU want to be swimming around and breathing in your toilet? I'd think you'd want it clean if you had to.
The rules for fish keeping are simple for success. Cycle your water. If you are setting up a new tank, start with Tetra Safe Start. No matter the size of the tank. Each tank has several naturally occurring chemicals in it. Nitrates, Nitrites, and ammonia. To seriously understand this takes some reading, but basically ammonia is BAD and will kill your fish. Nitrates and Nitrites should be present in some degree. When you pour tap water into a tank, and add fish, the ammonia levels will spike as there is no natural filtering going on. Once that happens, your fish is going to be at least sick, at worst dead. It takes about a month, or a little more to build a cycle in your tank. Tetra Safe STart helps jumpstart the process, though the best way is to take filter media from a tank that is already up and cycled.
Once your tank is cycled, never 100% clean it. Washing all that gravel and filter media destroys the cycle and you must start over from scratch. Instead, change 25% of the water, vacuum the gravel a bit and add it back in, using a water conditioner such as Stress Coat (my favorite). Live plants help the cycle too.
Bettas, are pretty carnivorous. I find mine LOVE frozen bloodworms, find them at your petstore in cubes. A few bloodworms are a great treat. I also feed mine Hikari Betta Bites, 1-2 pellets a day.
And for those, who ask why is this all important, we're talking about a 3.00 fish. Whenever you bring a pet into your home, you are taking responsibility for it's care. It's up to you to make sure the quality of life is the best possible. A 5 gallon tank doesn't take up much room, and really is easier in the long run. With a heater and filter, a few plants and a hiding place, your fish will have a much better quality of life.
Bettas are wonderful pets. Even with all the fish here, they and my goldfish are my favorites. And just be grateful you didn't GET goldfish. They need at least a 10 gallon tank per fish. Though there is nothing prettier than a properly maintained, properly sized goldfish tank.
There is a great forum called
www.fishlore.com that I'd highly suggest for those who want to learn more about how to best care for their fish.