hey there, that's a textbook magnesium deficiency, you can tell by the serrated edges yellowing . It's due to the heavy light schedule and the perfect environmental conditions meaning the plant goes into overdrive and needs more magnesium for photosynthesis. the problem will get worse for sure, unless you reduce the light stress but cutting down the light schedule and keep adding cal mag, also as a spray twice a week just before lights on 1ml/L . Hope this helps ! ๐