Is this fin nipping or fin rot??

I woke up today with a rather large chunk of my bettas fin missing, I'm not sure if it is fin nipping or fin rot, or his fins got torn on something in his tank. Please help I'm not sure what to do and I don't want it getting any worse.

(I'll add some before and after pictures and I also wanted to mention his tank mates are 3 neon tetras (4 of them died) and 2 Cory's)