Betta fish are poor tank mates, as they are prone to fish-on-fish violence. Even mating can be a physically demanding affair that leaves the female injured. The mating process is a multiple-step ordeal that starts with the male creating a place for the two to enjoy some privacy and get to know each other.
Building a Nest
When the male is aware of a female's presence and he's ready to mate, he starts by building what is commonly known as a bubble nest. To do this, he intentionally gulps surface oxygen and releases it under the water as bubbles coated in mucus to prevent premature popping. After several hours of this, he'll have constructed a small nest of bubbles inside the water, which will become his breeding ground -- as soon as he can lure a female inside.
Chasing a Mate
Getting a female into the bubble nest isn't always as easy as it sounds, especially for these stubborn, violence-prone animals. If the female resists going in the male's bubble nest, he won't take "no" for an answer. He liable to chase and bite her until she goes inside. The female may even be visibly injured by the time she's in the nest, with actual pieces of her body having been bitten off by the male.
The Egg Drop
Once the two are inside the bubble nest, breeding begins. The female's breeding tube has now emerged; when the two fish embrace, she releases her eggs into the water. As they sink, the male pursues and fertilizes the eggs. He then gathers them to carry to the bubble nest, while the female may remain there or make her way out before the male returns. Either way, though, she won't be staying there for long.
Protecting the Eggs
The female may feel compelled to eat the fertilized eggs, so if she's still in the bubble nest when the male returns with eggs in tow, he'll chase her away or even attack her without mercy. Once she leaves, willingly or otherwise, he keeps the eggs safe in his bubble nest and personally guards them until they hatch 24 to 48 hours later.