My first go (grow)

These last two weeks the growth has been explosive. Gained nearly 18" on plant #1 and about 10" on plants #2/#3. All the plants have become extremely bushy and completely fill the grow space such the the leaves at the canopy are impinging hard against the walls . The grow space is 37" x 17" x 80" . I am almost out of head room to move the lights up any further. Moreover, the canopy is so dense that no light is reaching the lower parts of the plant. I have added to 2 x 26W bulbs to the light fixture. I am amazed at the number of bud sites :) I have 2 concerns at this point. Plant #1 has lost about half of it's lower foliage do to yellowing of the leaves . Plants # 2/3 have had a few losses but nothing like #1. My first suspicion is that the lower leaves are just not getting light. I have tucked leaves like crazy up top but the only way to correct this would be an aggressive defoliation, I am loath to do this for a variety of reasons especially with me losing many leaves on their own. I am also bumped up the nitrogen and iron a bit on plant #1 to see if that helps. PH is right on target at 5.6. Concern number 2 is that I have caught a black fly or two but I will be damned if I can find an aphid despite a very thorough scanning of steams and leaves ...