first thing always check that ph, not just what you put in but check what comes out the bottom. if ph is ever the problem its 1 of the eaiser things to fix. some people freak out and start dumping diffrent types of additives or they start cutting back on things. its ok for the bottom leaves to turn yellow. just make sure it dosent start making it way up your baby. i end up cutting a goog chunk of the growth at the bottom anyway. this helps increse circulation and also makes your baby focus there energy to the top cola or colas. remeber they are a weed, they can take some abuse they are very hardy. good luck with grow i`m on my second grow. i used to worry about every change but i have learned that some of the changes peolple freak out on are normal-peace