Originally Posted by mewhales
speckles is an extremely docile betta and she have always gotten along with Ninja and Liliac. its just that Liliac and Ninja have an issue which i don't even know. i really don't wanna return Liliac if i have to. i might just buy another female just to spread out the aggression.
With thirteen gallons I would suggest buying two more, so which will add up to five. The more female Bettas in a sorority the less aggression there will be because they'll spread out, make sure your tank is like a dense jungle with either live/fake plants. Also hiding caves.
You might want to cup them all when you get the new girls and wait a few hours and release them at the same time, they'll establish a pecking order and one will become the queen bee, most likely Liliac since she's the most aggressive.
But yeah don't buy just one more, buy two ~ four more.