I use tap water with conditioner. I let mine sit out for 24 hours to go "stale" and let the chlorine dissapate. Even if you do that, you still have to use water conditioner as many municipalities use chloramine instead of chlorine and that does not dissapate through evaporation.
Though, I don't know if heavy metals are an issue for bettas, as they are for snails. Maybe someone with more experience can chime in. We don't have alot of heavy metals in our water, but I know some areas do. I don't know your situation.
I'm on city water, not a well.
I think we'd need to know more about your water supply to give a more difinative answer.