If your 100% sure about your ph and ro which you seem to be then I'd agree with the pest issue. I've had a gnat issue in my current grow from using organic soil so when I bought the yellow sticky strips for those I also bought thrip strips because I noticed on a few leaves there looked like something was eating at them. Google thrip damage to leaves to see if it matches up with your current issue. If so here's what I use maybe it will help you. You can use stacks or even twist ties if your plants are large enough to hang from the branches.
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