Week 6 flower update, chasing a lockout in DWC

Running a 4x4 DWC setup under a 600w HPS paired with a 200w Samsung QB for supplemental blue. Week 6 of flower on a Gorilla Zkittlez cut i've had for two runs now. Reservoir sits at 68F, pH locked in at 5.85, and I bumped EC to 2.4 (roughly 1680 ppm on a 500 scale) coming into peak demand weeks. Things looked dialed until about day 40.

Started seeing interveinal chlorosis on mid-canopy leaves, classic magnesium deficiency pattern despite the fact that my cal-mag is already sitting at 4ml/gal. Initially i thought it was a pH drift issue, but my Apera was reading consistent 5.8 to 5.9 on three checks over two days. That ruled out lockout from pH pretty quickly.

Eventually traced it to EC creep. The plants were drinking water faster than nutrients, so the reservoir concentration was climbing toward 2.7 EC between top-offs. That narrow window between 2.4 and 2.7 was enough to start antagonizing Mg uptake at the root zone. Dropped the rez, diluted back to 2.2 EC, added an extra 2ml/gal of cal-mag on top of my existing dose, and the new growth is coming in clean after about 5 days.

Still watching it closely. I've got a float valve on order to automate top-offs with plain RO water, which should prevent the concentration drift between reservoir changes. Anyone else running high EC in late flower and dealing with this pattern? Curious whether others adjust top-off strategy or just do more frequent partial reservoir swaps to keep things stable.