Just passing by here and saw this. I had a similar betta before, it didn't want to eat dry food and pellet. I try feeding it frozen blood worm, and it eat like starve for 1 week. Frozen blood worm can be found at most of the aquarium shop in KL.
Betta not so choosy about water quality, it got labyrinth and can breath from the air. And the way you change water is correct, salt is not needed.