Welcome to the forum and sorry you are having problems......
This is the main reason its a good idea to QT new fish........
Right now-I would get the Betta out of the tank and into a small QT container and start high dose aquarium salt treatment for 10 day....
Treating the main tank is going to be more of a challenge since those species can be sensitive to both salt and many medication...Right now I would keep doing daily 50% water changes and get the water temp in the low normal...76-77F
For the Betta only-
Premix some treatment water in a 1gal jug of dechlorinated water-Add aquarium salt 3tsp/gal and a tannin source if you have one-either IAL (1lrg crushed/gal) or dried Oak leaf (20 crushed/gal) Let this steep for 30min so that the tannins will start to release and salt to dissolve....Shake well
Using this premixed treatment water make 25% water changes every 15min for 1 hour on the Betta in the Qt container-today-this will get him acclimated to the new chemistry and start the treatment.
He needs to stay in the covered QT in this treatment water for the duration of the 10 day treatment period.
Temp 76-77F-float (attached so not to sink) in the heated tank if needed.
If you added a tannin source-the water should look darker every day
Tomorrow-using the treatment water-make 50% daily water changes for the next 9 days to complete a full 10 days of treatment.
Nutrition-good quality varied diet fed in small frequent meals-If you have access to mosquito larva offer several rinsed for 1 meal a day.
Good luck and keep us posted.....