It looks like a Nitrogen lock out, but if it is I am surer other plants would show it as well. A good pH number for Veg. is around 5.5 or so. Flower you will want it to 6.2
One other trick you can do to slow down the growth of a stem is putting a knuckle in it. What I do is pinch where I want the knuckle and bend the top part of the plant down. The plant will repair itself and the stem will be stronger. I will take a picture to show you what I am talking about
I see a lot of clipping in your near future. lol. Your plants will get big and grow fast. I grow in hydro with the clay pellets. In Veg. I run the drip system 24/7. I helps keep the pH in check as well.
Be careful with the center pot. You don't want to put too much solution in that causes it to overflow. I think you will be happy with the air stones in your rez.
I would put the rez outside of the grow room if the temp on the outside of your tent if possible. You don't want the solution to get too hot, above 85 degrees. The will cause you to have root rot. Put a air stone in the rez as well. You can't have too much Oxygen in your solution. It will help...