Condition before the water touches the bettas. You can either do it in buckets, or cup the fish while you fill. I used to not bother most of the time, but when the fish is sick... pretty much every bit of stresser counts, even temporary diluted chlorine exposure. The issue with poor water quality and stress is that it's never just nitrate poisoning - it's also whatever bacteria or parasite your fish loses its immunity to. I wouldn't rely on just water changes for the sickest ones. Stress from heirarchy imbalance doesn't kill this fast. Unless there are violent problems, the heirarchy would balance out before serious issues like dropsy. Stress could contrubute, sure - but the reason your fish are dying is mainly because of something biological going on in the tank, not just stress, or stress as too significant of a factor.