We don't have the huge boon that is soil when doin hydro-tek...we have tighter margins everywhere and more work to do every day. The ultimate organic soil growing takes 3 touches maximum a flower other than watering.....while a basic hydro-tek bare minimum is going to have me touching my plants 3 times daily....
Your ph is swinging wildly. Now the question is why... swinging down is typical with organic matter. A simple fan leaf being in your filter can cause a convergence somewhere near 4... So the amount of root mass seen in the photos could easily get your ph that low, a few dozen times over for sure.
Your ph problem is wither your water source or temp IME, which is related to ph but pH IMO is not directly the cause of your issue. Ideally youd be drifting up towards a maximum near 6.2 or so after touching, not drifting down, You're coming in at the bare minimum for absorption of any of the 16, then only going lower. Straight up, expect them to turn (especially the one) any day now. A finicky plant can easily be convinced to cull some root mass with a 4 ph or hot water.