Betta Fish: Betta has some black on the tail

Hello guys!

I recently got a betta from the store. He was almost dead in a cup.

I have seen that he has some spots on the tail. What could it be and what should i do?

He is in a 54l (15gal) tank, heated, well filtered, cycled. One filer also contains already cycled sponge from another tank.

Is he healthy? Does he look okay?

I also plan on adding more plants.

Thanks!