Flame has fin rot - Quick help please!

[DogFish]

After I showed pictures here of Flame about 10 days ago, I was warned that because of his chewed tail he might develop fin rot. Well this morning I noticed a piece of dorsal fin splitting off, and this afternoon it is much worse. So I am off to get some Betta Fix. He is in a 10 gallon by himself, the ammonia and nitrites are zero. I put in a few new plants yesterday. I do 40% water changes twice a week, so I believe the tank is very clean. He gets Hikari betta pellets about 6 a day with one day of fasting.
1. Could he just be developing fin rot now after 10 days? Or could it be something I have done/not done? ie the new plants?
2. Can I treat him in the 10 gallon? I have a 5 gallon that is supposed to be my hospital/quarantine tank, but I don't have a heater or filter yet for it.
3. Is the bacteria in the tank? If I need to switch him to a smaller tank for treatment, can I simply switch him with one of the bettas in a 2.5 gal (My gut feeling says don't do this).
Help!

 

[Tempestuousfury]

I wouldn't switch him with another fish because the other fish might develop fin rot. The fish is ok where it is right now. You're going to have to use more meds unless you have another container in which to place the betta. Add some salt and just keep doing what you're doing.

Plants don't harm the fish.

 

[DogFish]

I am off to the lps, do I really need to do a full water change?

 

[Tempestuousfury]

No. A partial water change should suffice. 30-50% might be a good idea before adding meds.