Hi
@GoodGoodWing! Hard to say without closeup pictures, but it sounds/looks like Manganese deficiency. Too much Magnesium, or a wrong pH range can lead to this kind of issue. I see you're having a 6.5 pH, which is good for soil but not acidic enough for hydroponic/soiless medium, so I think yellowing occurs because of the wrong pH. As an emergency move I'd try to decrease the pH to the ideal soiless range (~5.5-5.8) and if this doesn't get better after a few days, try to feed with a foliar spray and the fertilizer of your choice (containing Manganese). Hope this will help, keep us up-to-date, and happy growing 👊