Ph ? Nut ? Not sure what it is on the excess/defiency chart. Details below

In soil you want your ph 6.5-6.8 in soiless I run 6.0-6.3. The yellowing on your plant is at the top of the plant so we know its not N deficient, Iron deficiency will cause new groth to yellow, so will lights that are too close or too powerfull. PH is most likely your issue, unless you have lights way too close. For future reference mobile nutrient deficiencies start at the bottom of the plant, immobile will start at the new growth. Nitrogen is a mobile nutrient, which is why nitrogen deficiency show up on the oldest leaves first and works its way up the plant if untreated. Happy growing.