The first time I grew that happened to me too. The lower leaves start to lose light because the top of the plant is covering the bottom. It does look like you are overwatering a little because of the leathery type of feel of the leaves so you may want to cut back three or four ounces and add a little catalyst - kelp meal - molasses - humic acid. You can add supplemental lighting to the bottom portion on the sides of the room. I suggest fluorescent tubes or if you like CFLs those are good too but they get hot very quickly - almost as quickly as you turn them on. No worries.