A 1-gallon tank is the bare minimum size you can house a betta in, and it certainly isn’t ideal. Bettas are known as “siamese fighting fish” because in the wild, they inhabit large rice paddies and river basins where they have plenty of space to swim around. A 1-gallon tank is really only suitable for housing a betta temporarily, such as when you first bring him home from the store, or if you’re quarantining him before introducing him to a larger community tank.
In general, a betta needs at least 2.5 gallons of water to be comfortable and have enough room to swim around. If you want to keep more than one betta together, you’ll need an even larger tank – at least 5 gallons for two fish, and 10 gallons or more for three or more fish.
If you do decide to keep your betta in a 1-gallon tank, there are some things you can do to make it more habitable for him. First, get rid of any gravel or substrate – all that will do is collect waste and make cleaning the tank more difficult. Second, add plenty of live plants – not only will they help oxygenate the water (which is important in such a small space), but they’ll also give your betta something to hide behind if he feels stressed out. Finally, don’t forget to change the water frequently – at least once a week – and vacuum the gravel/substrate each time to remove any built-up waste.