IMO, pH and hardness shouldn't be overlooked, even with Bettas. We have very soft water with poor buffering capabilities where I live. Because of this, the pH drops dramatically within a 24 hour time frame. Out of tap it's around 7.8, but it drops down to 6. To keep it stable I use a buffering powder.
I had a lot of fish problems (Bettas included!) before I came to realize this. Thankfully, I was on a Goldfish forum and they recommended I test my pH out of tap, pH in the aquarium, and the pH after 24 hours. I was also told to invest in KH and GH liquid tests. Without doing those tests I would have never known our soft, poor buffered water, was the source of all my problems.
It is true they can adapt to a stable pH, but so can Neon Tetras.
I had a lot of fish problems (Bettas included!) before I came to realize this. Thankfully, I was on a Goldfish forum and they recommended I test my pH out of tap, pH in the aquarium, and the pH after 24 hours. I was also told to invest in KH and GH liquid tests. Without doing those tests I would have never known our soft, poor buffered water, was the source of all my problems.
It is true they can adapt to a stable pH, but so can Neon Tetras.