Second Grow. Problems going into 3rd week of growth

Right now, I have 7 plants going into their third week. Some plants are a few days older than others, but I have noticed that the younger ones are stunted growth and a couple of the other ones' new leaves are growing out yellow with little brown spots.

I am in a home made tent. From 6 mil Panda poly.

I have the plants in a DWC, for now. T5 light about 4 inches away from top of plants.
I had started by adding a small doze SUperthrive to water. PPMs at about 400 ppm.
Temperature is 79 degrees. Humidity 30%.

I stopped adding superthrive a week and a half ago, for I kept reading nutes really are not necessary at this stage.

So why do you think I am having these problems? Is it the nutes or the Ph?
I had noticed the PH was high (about 7-7.5) yesterday( I had finally gotten my PH tester kit, cause I had run out) and I brought down to around 6.

ANy suggestions?

PS: best way to accelerate root growth in seedlings?