betta with shredded fins?!

[succulent]

this morning, my fish had beautiful finnage with no sign of injury, but when i came home from school, i found gaston with an absolutely shredded tail fin. he has a sponge on his filter, no tankmates, a heated 2.5 gallon tank, and soft decorations.
what is the problem??

 

[succulent]

also, as you can see in the 2nd photo, only his top part of his tail fin is damaged.

 

[redfish54]

How long have you had it? Did you cycle the tank?

 

[TeenyFish]

Definitely nothing in the tank he could have got caught on?

What are your parameters? (Ammonia, nitrite, nitrate?)

He could well have done it to himself (fin nipping). Bettas are known to do this when either the tail gets too heavy, or they're stressed.
Considering you just got him, it could be the latter.

How long was he in the bag for from store tank to home tank?
How did you acclimate him?

 

[Jo26]

Looks like fin rot to me? I had a guppy that had it and it looked exactly the same as your bettas tail. Was advised on here that pristine water conditions clear this up and it did the trick.