While a betta fish can live in a 2.5-gallon enclosure, a 5-gallon aquarium is ideal. Make sure to avoid overcrowding any betta fish types, as this can potentially be a significant cause of stress and disease. Also, be sure to provide adequate filtration and maintain high-quality water conditions with regular water changes.
The best way to introduce two betta fish together is first with an adjacent tank. This is a separate tank outside the aquarium or inside the aquarium with a divider. This way they can see each other without touching each other and you can see how they interact. Watch how they behave together as it can take a little time to learn to get along.
Find more information on caring for your pet in our guide, Betta Fish Care: How to Take Care of a Betta Fish.