Root help

I have three plants, 3 weeks old and 4 - 5 inches tall lots of leaves vry bushy, ppm367, ph6.3 , temp 78 degrees, 400 watt metal haylide light 14 inches away, the roots in the net pot look nice and white, but when they extend out of the pot they only last a day before turning brown and dying off, maybe i need to back off the nutes till its around 250 ppm, i dont know please help