Bettas may dislike the company of other fish, but that doesn’t mean they enjoy small tanks with limited space. They're inquisitive fish that love to explore and cruise around in larger bodies of water, and they’re much healthier with a clean and heated home. Tank size matters.
Two-Gallon or Larger
Any aquarium with capacity less than 2 gallons is not suitable for a betta. An aquarium of 2 gallons or larger gives your betta room to swim freely. Bettas love swimming around the edge of the aquarium, playing with toys, such as ping pong balls, and ducking into and out of plants. With a tiny tank, he'll barely be able to turn around, much less play with toys. Some stores sell tanks that claim to be specifically for bettas, but many of those tanks aren't even a gallon in capacity. Your betta will not only be unhappy in a tank that size, but she'll also likely have a lower life expectancy.
If you want to shoot for a larger tank, you can stock it with more than one betta, but only if you add a divider. Most commercially available dividers are for tanks 10 gallons or larger. Bettas don't enjoy the company of other bettas and tolerate few other types of fish. If they're placed in the same tank without a divider, they will fight. Even if you do add a divider, the friskiest of bettas may make a mad dash for the other side and leap over the divider -- so always keep an eye out for the sudden appearance of one betta in another's space.
Heater and Filter
The smaller your tank, the more difficult it is to regulate your betta's required water temperature that's between 75 and 86 degrees Fahrenheit. A 25 percent water change in a 1-gallon tank will result in a more drastic temperature change throughout the tank than a 25 percent water change in a 10-gallon tank. Many aquarium heaters are not suitable for tanks under 2 gallons, because they may overheat the water and kill your betta. Bettas hate powerful currents, and even an adjustable flow filter set on low will result in a strong current in a small tank. In a tank with a strong current, the long flowing fins of a betta may become torn either by the current or from being pulled into the filter intake. Additionally, tanks of capacity less than 2 gallons are already limited in space. Add a filter and heater, and your betta has significantly less room to swim around. Some smaller tanks may not even be able to accommodate a heater or filter.
Plants and Decorations
While your betta will happily swim about in a barren tank, he'll much appreciate a few plants and decorations. With a tank that's too small, there's barely room for plants, let alone enough space for decorations. He'll use the floating plants to build bubble nests, although they won't serve any purpose if he doesn't have a female to mate with; and he'll have the rooted plants to swim about and rest in. A large decoration or two gives him a few spots to slink into when he's tired. Don't crowd his space too much, though: He'll enjoy plenty of free swim space more than a tank littered with plants and fixtures.