Betta tank size is a controversial issue. On a purely physical level, Bettas have an efficient digestive system that doesn’t produce much waste, plus they breathe air, so they don’t absolutely need a larger space. In the wild, they basically live in small, stagnant sections of rice patties and other pools of still water and mud holes.
On an emotional level, however, things are different. How happy would you be if you had to spend your whole life in a tiny apartment? Probably the most convincing argument for me though is the fact that Bettas will always use the full betta tank space you give them.
One Betta I got, Charlie, was sad and lifeless in the little vase he lived in when I got him. He was sickly and lethargic.
Then I moved him into a 10-gallon betta tank and today he’s in hog heaven. He spends his days racing and frolicking through his water, exploring pathways through his plants, and flinging around his gravel. He enjoys life and doesn’t have to sit crammed into a vase anymore. Vases are for flowers, not Bettas.
So, even though your Betta would be okay living in a gallon-or-so-sized jar, there is no reason to consign him to a life of confinement in a Betta Tank. Bettas will thrive in a goldfish-sized bowl, a small aquarium, or even a larger aquarium under certain conditions.
If you decide to keep your Betta in a small container, remember this:
Although Bettas are used to living in shallow mud holes, rice paddies, and swamps, these habitats are part of a natural ecosystem. This means that the water receives nutrients regularly, freshwater constantly flows in and bacteria are flushed out and destroyed by the natural water purification process. And the typical rice paddy is enormous — about the size of a large pond.
None of this happens in an artificial environment, and the smaller your Betta’s habitat is, the more danger they are in due to the quicker formation of adverse environmental conditions.
Overall, your Betta will be happier in a small tank than he will be in a plastic cup or vase.
For your little ones to not only survive but to start to be happy, give each Betta a bare minimum of 2 gallons of water.
And really I’ve found (and so have the other breeders I’ve interviewed) that no Betta — male or female — will ever get upset with too much room. There’s no real upper limit to how big your Betta’s tank can be, except for the practical fact that larger tanks are that much more difficult to heat and keep the water clean. So consider the maximum (and ideal) per Betta to be about 10 gallons for each fish.