Is my betta tail biting?

Hi! I thought my betta had fin rot and treated him with Bettafix and aquarium salt in a small QT tank. I took his 8 gal. and cleaned it with extremely diluted bleach and rinsed and drained the tank a dozen times before starting a new cycle. He was in QT with aquarium salt for 7 days following treatment directions.

I've never noticed any black edges on his fins, just that he was slowly turning I to a crown tail 😁 it never looked like it melted away, it was just really choppy. The thing that tipped me off today was that it's just on the fins behind him, not the ones on his belly, the two that trail, it his dorsal fins. I was reading up on fin rot not healing because his tail is trashed now, and I found information about biting.

Tsunami is a half moon in an 8 gal. Aqueon Evolve tank. I had the filter that is built into the tank running with a binder clip in front of the filter to break the flow. However, the clip began to rust, so I replaced it with a balloon with holes and eventually turned the pump all the way down and ran it in hindered in cycles. Tsunami would be knocked around periodically. I then got a filter that uses an air pump to bubble air through the filter. I forgot the name of it!

I'm attaching a picture of Taunami and his tail. Please give your opinions as to whether it's rot or biting!

8 gal. tank
No tank mates
Heated to 78-82°
Filtered
Air stone
Two plants
Day and Night light settings