From the picture your plants look great! I have never been a fan of r/o systems because they over purify the water. I have installed a lot of water purification systems over the years and the way it was explained to me was that an r/o system completely strips the water of all micronutrients so when you drink it, it will actually pull nutrients from your body to stabilize the water. If that is true then I would imagine it would do the same thing to your plants. Could it be possible that you might have root issues from the soil not drying out enough? That seems to be the biggest issue I have over the years. I really doubt that water quality is your issue because if you see the water I pull out of my pond to use it can be pretty ugly in the summer time and I don't have any issues. What your are describing (imo) sounds more like an over watering issue. Even with coco loco I have to back off my watering sometimes. Hope this helps!
To be honest and fess up I thought I would give Fox Farms "Strawberry Fields" blend as it read to be best for veggies and flowers. Surely a rookie mistake going all in on that one. I should have split it up with different soil blends for a more broad experience.