Leaves going black and drying out

I can’t tell if you have all sand in your pot or if you only have a top layer for gnat control. I hope it’s not all sand. Either way it looks like you may have shocked the plant by suddenly changed pH / over nutrient. Did you pH your solution before feed? Your target should be roughly 6.3 pH. Your nutrients at that stage should be roughly .6-.8 EC. If you did not pH your water then I strongly suggest doing that and running a good flush of pH balanced water though your pot. I hope this helps. I hope this helps.

Im no expert i've never seen weed grown in sand medium. Sand has zero nutrients that the plant needs so in order for it to grow into what you want you have to provide it with all the ever changing nutrients that it will need to support its stages in its life cycle. Also sand packs in really tight after watering and weed needs loose soil to absorb some oxygen. Change the soil to actual dirt and I think that will fix most of your problems.