the yellowing leaves that are falling off ,dont get too worried about it ,you will find that some of he plants leaves will fall off ,the yellow colour is just that the plant has been taking all the nitrogen from that leaf and its the plants natural way to discard the used leaf ,this is perfectly normal try not too worrie ,the only other thing that could cause yellowing is a magnesium deficency which is curable with epsom salts
cheers all