Brown Roots HELP!!!!!!

I'm growing in a drain and flood system using the lava rocks. I have the plants in grodan cubes and watering once a day. The plants are about 3wks old and the roots are brown. For how old they are the root are about 9in long. What can i do to get the roots healthier? I have added a 4in air stone yesterday is this enough?
I am a first timer to so I need all the help i can get.

flush flush and more air. what kinda nuts are u running and how much. if i were u i would fill my res with PH balanced water and flush for 3 hours or so and got with 1/4 strength nut solution for a few days and bump it up to half for a week and see what happens. Flushing it for a few hours each week when u change ur water will help and if u wanna try some hydrogen peroxide with yer solution about 1 teaspoon per gal