Leaves yellowing

If you're using a proper living soil I don't think you should be needing to ammend anything this soon. It should already have plenty of stuff available for absorption. Those first couple weeks the plants don't really require much and really they're only start to really feed now. I'm a little concerned about the other things you've added, specifically the oat flakes. Material like that needs to compost before you grow in it. As it breaks down it's affecting soil pH and even soil temps. If you added a lot of oats your roots might be starting to bake. I think RO is a bit overkill in a living soil grow. My tapwater is super hard(little over 400ppm) and I even use mine in hydro applications. I seriously doubt it was the cause for major issues. Sometimes you might run into some minor micronutrient issues using hard water but most decent fert lines also make micros formulated for hard water too. Some fert lines "require" RO, Athena is one that immediately comes to mind, but a soil application relying on microbes shouldn't be that sensitive.