First grow, stem problem or paranoia?

Hello everyone, this is my first grow. These plants are 17 days old and being grown with an aero setup. When I woke this morning i noticed browning around the base of the stems, the rest of the plant looks otherwise perfect. The ph is just a little under 6.0. The nutes are hovering at 900 ppm. Do I have a problem? Or paranoia?

Thanks for your input.