He is very beautiful! And i love your bettas names! Oh and thank you for rescuing both of them:)
Just in case to be on the safe side do not share anything between them in case Acarnan has something contagious going on. The caudal fins are damaged, but looks like tail biting though. The blackened edges on the other fins looks like his normal coloration.
Do daily water changes . Stress coat is very good idea. If you can get frozen blood worms , it has a lot of protein and really good for the immune system. Also NLS has garlic in it and also really good quality pellets.
The best quality pellets are
About salt. I think if he is getting better with water changes do not use salt. Salt is very good and helpful when needed . But it also stressful on kidneys . So if he is getting better then i think he will continue to improve without a salt. Also at any time when you using salt it should be therapeutic dose . The therapeutic dose is 1 tsp/gall up to 3 tsp/gall. And just make sure you always pre mix and make sure its dissolve before you put it in the tank.
Give us update please;)