Just want to say that i agree with Lizzy and Syriiven. With tail biting extra clean water will help. I also have on of my bettas tail bitter and extra water change keep it from infection. And i agree on stress coat API it will help to restore slime coat. If you ever will want stress coat API and can't find-there is another brand similar to stress coat let me know i will check on name of it. I saw that Sakura8 recommended it .
And with Lizzy on medications. Bettas are sensitive when it comes to medications, as are most all fish... so it's recommended to be very careful when using any medication. Why most will tell you to try Epsom or AQ salt first as they are safer and easier on the organs.
And again if it tail bitting you don't need medications since you not going to resolve the problem. And you can't treat them all the time after they bit the tail. So really extra water changes with vacuum will the best and less water movement. Don't forget to rinse the filter and aquarium sponge