It is much easier to use a bit of your tap water rather than these chemicals. Your hard tap water contains the same stuff, so why spend money buying what comes out of your tap.
For a betta I would not use just RO water, I would prefer water for a betta to have a bit of hardness. RO water has no hardness at all, zero. So you need to add a bit of hardness to the RO water. Your tap water has lots of hardness, that's what makes it hard. If you use a bit of tap water in with the RO water you''ll be adding a bit of hardness. Yes, you can add it from a bottle but using tap water is easier and cheaper.
Use the machine to make some RO water. Then half fill your water change bucket with RO water and put tap water in to make it up to a full bucket. That will make water at half the hardness of your tap water, which is perfect for a betta.