yellowing tops

Hi, this is my first time to grow hydro. I'm growing in 1x1x2 Homebox with 2x600hps and i keep my temp at 27c-28c (on purpose). I have good setup for watering the plants (after many tries and re-calculations). I kept the PH arround 5.5 - 5.7 during veg and i have let them veg for about 6-7...