Oh jeez GK, ive been reading some of your threads, your plants at 3 weeks are huge compared to mine,mine growing soooo slow, what could be wrong???

They're getting fresh air, temp between 78-84, 400w MH, pH is 7, water them every 3 days or when its nice and dry, soiless mix (1/3 coco,1/3 vermiculite,1/3 perlite), only given them fert once (1/4 strength), they getting 18/6 light

I dont know what im doing wrong!! '
Please help