When you get a new fish, plant or invertebrate and you want to add it to an already established tank you need to quarantine it ( put it in its own tank). You should QT things for at least two weeks before adding it to a tank, otherwise they could bring diseases and parasites into your tank.
Flaring is a normal part of bettas, their kinda known for having their big grumpy attitudes.
Feel free to message me any questions, I'll try to help the best I can!
Rummy- Mini Rex Rabbit
Diz jr- Betta
Yoshi- Crested Gecko