Problem with leaf edges curling up

I have three plants that are 3 weeks in veg under a 150w HPS in a cooltube. 2 out of the three plants have some leaves that are curling upwards on the serated edges. Now my first thought was heat stress but my temps at the canopy average about 76 and dont go above 80. The humidity averages 40-50%. I really have not seen this before and dont see anything in the sick plant guides. Any opinions? Thanks

so am I asking a stupid question...is there something obvious im missing??

You are asking a reasonable question. My plants are doing a very similar thing. I am growing in a mix of 50/50 perlite and Canna Coco. I keep the PH between 5.7-6.2 usually exactly 6.0-5.9. I am doing a drip system and under 1k watt HPS. I am using GH 3-part with Kool Bloom, Florolicious Plus, Carbo Load and Overdrive. I am keep the ppm below 1600 and the girls are about 12 to 22 inches tall about. Temps are between 56 and 77 Degrees F and humidity between 30 and 60 %. Any recomendations would be appreciated.

Typically, that's indicative of heat stress, but based on your temps, that can't be it. Only other thing I can think of that might be causing them problems is that it's just not humid enough. I don't think it will cause you too much of a problem later on though as the plants get larger and put out more moisture.

Thanks for your response. : )

Humidity/Light problems.