if you can check the ph too. I would have thought it to be normal but if its only 8 weeks then it hasent (most likely) started to flower yet. And if you dont know during the flowering stage its not un-normal for the big leafs to turn yellow then die. It just the plant using all the stored energy for use during the flowering stage. Let them die and dont pluck off. Sorry if you already know this, just thought you might want a heads up bout that. And it seemed related, but then again who knows cause im stoned, lol.