Looks like lack of magnesium. At the stage your at now, If your using RO water. Id Bring feed to around .8 at the stage of growth your at. That would be .3 to .4 EC of calmag and the rest your nutrients and bring PH to 5.8. If your using tap water, id see what your starting water is because if its a high EC to start, youd have to dilute your water with distilled or RO because you wont have room to play with nutes until they get a-lot bigger. If your starting EC is below .3, id add some cal mag prior to nutes. Also id bring light to around 200-400ppfd if your using LED and not already in that vicinity.