I think you should start a new topic about this in the Betta Habitats section, you'll probably get better responses. If you're mixing RO water with the well water, you should have no issues. Bettas can adapt to hard water, and bacteria may be a little slower getting started, but they will still colonize the tank easily. After all, people with African Cichlids have cycled tanks. :) Just make sure the betta is acclimated very slowly with each water change, since the mixture is never going to quite match the old mixture. It still might be a good idea to use dechlorinator, it removes some heavy metals and other toxins from the water. And it's good to have around if you ever sterilize a tank with bleach or potassium permanganate.