I see no difference between softened water and RO water.
For the life of me I have a BBA problem and cant figure out how to get rid of it. I figured it was due to the high KH in the softened water so I switched to RO for a year long experiment.
I noticed no difference in plant growth and the BBA wasnt phased a bit either. I just switched back to softened water a few months ago with nary a problem (other than the BBA is laughing at me behind my back).
When I used RO water, I remineralized with GH booster and baking soda. Baking soda is Sodium Bicarbonate so I realize I was reintroducing sodium back into the tank but it was a miniscule amount compared to what was added by the softener.