The first two leaves are cotyledon leaves. They will naturally turn yellow and fall off.

Did you start with tap water? Are you using GH's PH adjustment chemicals? Are you testing PH with a meter? If so, make sure it is calibrated. When you add the ph adjuster chemical, sometimes you have to let it circulate for a while to get an accurate reading.

GH's PH up and down is a must have. I didn't have any the first week i had my system set up and my PH was all over the place. Tried natural fixes but they only worked a short time. I suggest getting the GH Kit that has the calibration fluid as well. Works great and last a long time.

I broke off a piece of one of my tomato plant stems and stuck it into a net pot so that the stem hit the water about 4 weeks ago and i have already a bloom spring up. I kept my PPM between 650 and 880 until just recently as i am running it around 1200 to 1600 ppm now. PH stays 5.8 to 6.4. I have 95 plants in my NFT system and it seems everything is working just fine together.

http://youtu.be/hB-FGkTpoWM

Here is a link that might be useful: Yesterday Video