some info on your setup and how often and how much you water and how often and how much do you feed and whats the npk of the fert your using? does it contain trace elements? dont remove those leaves, the plant is draining them of the energy it stored in them because it needs more n most likely so its killing the old growth to put into the new growth. if a leaf doesnt come off with a gentle tug let it be, your plant knows what its doing and when it is no longer worth keeping around.