Can male and female betta fish be kept in the same tank?

Female Betta need to be in groups of four or more. They are aggressive and if you only have two, they will likely kill each other. If you have enough of them, the aggression is spread out and they can be relatively healthy and happy.

It is only a myth that female bettas will not fight each other - they are very territorial and can be as aggressive as the males. One will try to dominate the other. Try introducing them after a water change, and try to put them in the tank as closely to the same time as possible, so they don't become too territorial. It is not recommended to put them in a group of three as two may team up and pick on the third fish.

If you notice them fighting, just create a water current manually, as this will discourage the behavior, and after two-three days they should get along fine.

Provide lots of hiding spots for them! This will allow them to hide and rest when they need to.

Never mix male and female Betta outside of breeding a pair. Also, never tank multiple males together because they will kill each other.