My Betta (Siamese Fighting Fish)

Because I have fishes for pets, about a month ago, a friend who is interested with bettas asked me if I do have any idea about them because he wanted to have them as pets together with his tortoise, during that time I don't have any interest in taking care of bettas since you can't group them or place them in a community tank.
Since I know where to buy cheaper fishes and its just a block away from my place I volunteered to buy for him and temporarily take care of them until the weekend. But after a few days of watching them, I decided to buy some for myself because they are so colorful and beautiful to watch.
As a rule in taking care of bettas, you should never put male bettas together for they will fight, sometimes to death so I placed them individually on empty soda bottles. But later on, I feel pity for them because they can't move freely so I decided to transfer them to my aquarium, and as of now I have 16, six males and ten females all in my 20 gallon aquarium. At firts, they are always fighting but bettas are territorial fishes and after establishing their owmn places, they are getting quite well now.