hey there JiggleB, i gotta disagree with my growmie on this one, this is the start of a magnesium related issue. Two things are happening, it could be you're giving too much Mg and you're seeing lock out. or more likely the light schedule is too intense and the plant needs more Mg to regulate photosynthesis than it can absorb through it's roots. You would do well to shave off a few hours of light to treat the root cause. But if you dont want to do that another thing you can do is make a cal mag spray just use 1mL/L calmag , spray twice a week. Absorption by the leafs is 80% efficient compared to 20% efficient by the roots. Hope this helps ! π