dark

I saw my Betta's fins like this this morning, Wondering if its fin rot? Showed this picture to the Fish Care girl in Petsmart she said my fish was just old but i have only had him 3mo :-( his fins are normally white/ light blue and separated but they have seemed to be fused together now

Whenever I leave him in my room with the lights off & I come back in and check him, he has stress stripes ): Once I turn on the lights for about 30 minutes he's back to his normal bouncy self with no stress strips. However, I am worried about him because he never really acted like that until I...