I'm a Siamese Fighter expert
Do what I told you and your fish will be happily and healthy ever after.
Add some real plants in there, 20% water change once a day for 1 week and a bit of salt and if the condition still the same then add a few more grain of tonic salt the next day before changing water. If you can find old sun dried banana leaf then he'll appreciate it more than Indian almond leaves. Banana leaves will make the water kind of smelly but that is normal
I only had fin rot once when my betta fry really small. It has ich and then fin rot but after I eliminate the problem my betta kind of immune to it until today.
So yes my fish never have any fin rot
I have lots of Java Ferns, Java Moss, Amazon Sword, Indian Ferns, duck weeds, Anubias nano and anna and some other floating one.
Cause of fin rot may be due to stress or immune system of the fish is getting to weak. Always feed bloodworm at least once a week or live foods even better. Feed variety of foods to keep him healthy and less picky later on.
Bettas doesn't really like too much light. Usually they hide underneath the lotus leaf, water hyacinth, water lettus, oxygen weed and floating dead leaves in their natural environment in South East Asia. And usually the longer fin Siamese Fighters are prone to get fin rot easily. The Plaket or wild one won't get fin rot that easily.
OH! Most importantly I have more than 20 Bettas in each tank currently. Before I had about 100-150 per tank, yes it is a lot, all that is from one batch.