Is this fin rot?

Hi! Is my betta having fin rot or other serious problem? I am still beginner and learning, and I have partial color blindness, making it harder to identify symptoms. Really hoping to take action quickly to make him better.

Picture from first few weeks i got him:


Pictures now (there might be some lighting difference):


Background:
He is in a big tank with plants, he never wants to eat pellet, only live worms and a little bit of daphnia that exist somehow in the tank. Very active, likes to swim and hunt.

Last week after coming back from 3 day vacation, i found some plants melted and the water condition was not optimal, and with that, i also found my betta’s dorsal fin and tail damaged.

Now 1week after the vacation, water is clean, his dorsal fin seems better or same, tail seems worse, and overall he seems slightly less active than the usual (this last point could be just an impression).

My several hypothesis on the damaged fin:
1. He hurt himself from hitting tank decoration, or
2. Fin rot from poor water quality, or
3. Ammonia or nitrites poisoning (unfortunately i havent been able to do the test)
4. Other reason worse than the two above

Could you please help suggest what i should do? I am thinking to move him to a smaller tank first and see if he makes progress, if not then I will treat with Blue Methylene.

Thanks!

You could try buying some aquarium salt. Just follow the directions on the back to see how much salt to add to the water, and be sure to dissolve the salt before putting it in the tank or it might burn his gills. Good luck!

Is it quite a new tank? You could try increasing how often you do water changes to improve the quality just in case that's affecting it. I'd also use aquarium salt and methalyne blue.

Just be aware that methylene blue will damage the bacteria colonies. And it will stain everything in the tank.