Help determine why plants are yellowing

I water until slight runoff, and make sure there is no standing water in the tray it sits in. I water, let sit and dry 24-48 hrs and water again when pots get light. My PH runoff was 6.7. Should I flush with straight water next? I am going to try to give tap water that's been sitting out for 24 hrs next, as I usually give filtered brita water. Maybe it is filtering out some iron or trace elements the plant can use and isn't getting from the Flora Series nute trio.