Leaves are curling up at the tip but why?(pics included)

as far as i can tell my plants are healthy going into 3rd week of veg. under 150 watt hps in soil water ph to 6.0 - 6.7 (i dont have my digital meter yet but this test is pretty good helped me out of my ph issue) using grow,micro,bloom as directed by advanced so as far as i can tell 900 ppm watering every 3 days or so i do allow soil to dry some before watering again temp is at 77.6 and humidity is at 55% the only thing i can think of is uping the nutes alittle so what do you think.....Thanks in advance

How soon are u watering? Maybe overwater. If leaves were curling up could be heat stress

It's nitrogen overdose. Here take a look at this, and all your questions will be answered.