light and dark green leaves

hey there grower eagle, i gotta disagree with Roberts here, it's for sure magnesium , you can tell because the discoloration is happening on the newest leafs first. That's because magnesium is an immobile nutrient, so it affects newest growth first, while potassium deficiency would start showing on older leafs first. Basically plants need a lot of magnesium when they do photosynthesis and you have a heavy light schedule. Conversely , these small plants hardly need potassium at this stage and potassium deficiency is usually a lot more colorful (red-ish) . Anyway, you do always need to grow with cal mag, even in soil, that's because the "high quality" genetics from these cannabis seed brands are almost always grown in lab conditions with extra calmag when they're selected. add calmag and the problem will stop spreading almost immediately and the plant will recover quickly. Hope this helps ! 🚀