A lot of people will argue that a 1 gallon tank is perfectly fine for a betta. Personally, I am not one of them. I'm not here to start an argument, and I know that if you want to keep your betta in 1 gallon, you will do so regardless of what I say, so I'm not going to try and dissuade you. However, since you asked, here's my response (as a person with a few years of betta experience).
A) Imagine living in a closet. It would suck, big time. Sure, you could survive, but wouldn't you rather do more than merely survive? As you released waste, it would quickly build up, and if the person responsible for cleaning your closet was even one day late, the waste (urine, feces,
CO2, etc) could build up to lethal levels. Even if it didn't kill you, it would be really unpleasant.
IMHO, a tank that's less than 2 gallons is like that closet. Ammonia and other toxins can build up to lethal levels quickly, putting your betta at risk. Of course you think that you will always be on top of your water changes, but sometimes things happen that prevent you from doing the needed 100% water change every 2 - 3 days. Do you have time for that? Will have be able to devote time to change 100% of the water in your fish's tank every 2 - 3 days for the next 2 - 3 years? Even if your fish only lives 2 years, and you only do 100% water changes every 3 days, that's still almost 250 water changes.
B) Bettas love to swim around!
IMHO, 1 gallon simply doesn't provide enough swimming space. I keep one of my boys in a 2 gallon, the smallest that I would keep a betta in, and I feel bad. He swims up and down the sides with his nose pressed up against the walls. If I could, I would not hesitate to put him in at least 5 gallons. And this is a tiny little betta! My normal-sized betta looked so cramped in there that I bought him an almost 5 gallon tank and he clearly loves it. Keep in mind that a betta grows to between 2 and 3 inches long. They're not little fish, and they do so love to have their space. If anyone tells you that bettas
prefer small spaces (and I know this is a common
myth), remind them that rice paddies are the size of football fields.
C) Bettas are very hardy fish, but temperature fluctuations can really weaken their immune systems, making them vulnerable to a whole host of diseases. A 1 gallon tank is much harder to keep stable than even a 2 gallon. Don't believe me? After my 2 gallon has had a 4 watt light on for 12 hours, the temp is 24º C. In the morning, after the light has been off all night, the temp is 22º C. That's a 2º C change. Toirtis has said that in his experience with a 1 gallon, there's a 6 - 7ºC change. For a fish, that's a huge difference!
D) I'm not disagreeing that bettas
can live in a 1 gallon tank. They can live in almost anything. I've heard stories of bettas living for years in coffee pots, tiny vases, you name it. This doesn't mean that they should. It's true that it's impossible to exactly reproduce a captive animal's exact environment, but
IMHO that's no excuse not to give them the best possible home. I looked on petco.com. That 1 gallon kit that Toirtis showed is $15.99. The 2 gallon version is $17.99. It's only $2.00 more!!!
Kate, I realize that I may come across rather strongly. I'm not here to offend anyone, I'm here to educate people on my point of view for the benefit of the fish involved. Do what you think is best for your fish.