does my fish have fin rot?

[Nuvolix]

I'm worried about my betta - I think he might have fin rot or something worse. As a beginner and with my partial color blindness, it's tough for me to tell what's going on. I really want to help him ASAP.

Here are some pics from when I first got him:



And here are some recent pics (lighting might be different):










My tank is pretty big with lots of plants, but he's super picky - only eats live worms and a bit of daphnia that somehow showed up. He loves swimming and hunting, though.

When I got back from a 3-day trip last week, I found the plants were all melted and the water was bad. My betta's dorsal fin and tail were also damaged.

Now, a week later, the water's clean again, his dorsal fin looks better or the same, but his tail seems worse. He also seems a bit less active (or maybe that's just me).

My top guesses for what happened to his fin are:
1. He hurt himself on a decoration,
2. Fin rot from bad water,
3. Ammonia or nitrite poisoning (haven't been able to test for it),
4. Something even worse.

Can you please help me figure out what to do? I was thinking of moving him to a smaller tank to see if he gets better, and if not, I'll try Blue Methylene.

[FinFan]

You could try adding some aquarium salt to the tank. Follow the instructions on the packaging to get the right amount and make sure to dissolve it properly first, or it might irritate him.

[hazely]

I'm thinking it's worth looking into the tank's water quality more closely. If it's a relatively new setup, it might be beneficial to increase the frequency of water changes - this could help rule out any potential issues. Using aquarium salt could also be a good idea, as FinFan mentioned, and possibly methylene blue as well, just in case.

[sSjey]

Keep in mind that using methylene blue can harm the beneficial bacteria in your tank, and it's also known to stain everything.