Too many people on this site think because a plant has a red stem more often then not it is probebly the strain or futher down the genetic line don't worry about it if only a few leaves are going yellow and dieing off its not a problem mine are in second week and i have yellowing leaves leaves do die.The leaves lower down are the first leaves to grow and the first to die take it easy have a bong and chill,I would try to lower your ph a tadd 5.5or5.7 till yhey are more in budd if it's a 8/9 week strain bang some pot ash on in thelast 2 week.