fin rot? Tail biting? What is it? How do I treat?

[mjbn]

wokeup to my bettas tail looking weird. Between the rods there were holes in his tail): idk if it's tail biting because he didn't bite his rods and it's only between the rods too. Idk what else it could be because there's nothing in his tank rightnow, just water..):ive been doing 100% water changes with .5 tsp/gal to get rid of the grayish color at the end of his tail but now it's that plus these holes): what do I doooooo?