I have extremely soft water with 0 kH so its buffering capabilities were non-existent. In 12 hours it would swing from 7.0 to less than 6.0, which was not only stressing my bettas but also wrecking my cycled tanks.
I use Seachem's neutral regulator keep my pH at 7.0. Even if I leave it for several days, the pH remains the same. The only budge in pH I saw was when I added three large Indian Almond Leaves into a 10 litre aquarium.
All I do is add some to my bucket during water changes and it keeps my water buffered. Don't do it suddenly in case it shocks your betta, but I've had less problems with fluctuating pH when using products to adjust it, than I had when I just let it be.
Personally I've found my bettas do better in neutral slightly hard water than they do in very soft water. I seem to have a lot less health issues now than I have had in the past.