Issues with Heat?

Howdy, this is my first time actually attempting to grow the right way. I have been reading as much as i can on it. Locally I dont really have anyone who knows what they are talkin about. My issue is with the sides of the leaves on a few of the plants curling up i have going. I have 6 of them under a 400w MH bulb for 24 hours a day that was approx 17in off the top of the plants in a potting soil bought at a nursery. I water them every other day with water that has approx 6 ph, and twice a week with Fox Farm organic Grow Big. The plants are approx 4 weeks old. Ive included a couple pics of the most extreme case in the curling. Any info would be greatly appreciated.

First off fox farm grow big is NOT organic, but thats not a bad thing I have been using it for years and I think it is one of the best veg foods you can buy. That being said are you feeding them with every watering and how much? I found about a half of a teaspoon per gallon when their that young every other watering works great. You might be getting nute burn if your overfeeding them, in that case you wanna flush your pots with plain water with 3 times the size of your pot. e.g. 3 gallon pot you wanna run 9 gallons of plain water through them and then lightly feed. Hope this helps

but the ones in the pic look healthy...

Huh, well i havent been feeding them with nutes everyday for about a week or two, i have spread it out to 2 times a week and im using 1 tbsp a gal before that i was using a 1/2 tbsp a gal. But u think they look healthy? I seen the curls on the edge, i was thinkin it was something to do with the heat burning them. I used to have the plants about 17in or so from the light source but once i seen the curling i pulled it back to 24in.

My recommendation would be to make sure and let them dry out before watering them, roots don't like to be moist all the time, they need time to reach out and search for water source...thus grow.

your soil may be deceiving you. the top layer dries out quicker when there is alot of wood in the soil. stick a finger in there for a better idea. http://www.marijuana-seeds.net/Thanks-ThankYou.htm

the soil is crap IMO, tomorrow im goin to a hydro store that just opened up and im gonna transplant 4 of the 6 with better soil, they have tons of different types of soil also. The guy who owns the place knows everything it seems. thx for the comments.

Also keep your ph at 6.5 - 6.8. At 6.0 you are too low and will eventually start showing Magnesium and Calcium deficiencies due to pH lockout. Just a heads up. You can also try picking up some "pulverized dolomite lime" while you're at the grow store, make sure it is not pelletized, calcitic or hydrated lime, read the bag first! Mix 2 tbsp of it with 1 gallon of soil and it will help buffer your pH and help prevent you from getting pH problems in the future.

Alright everyone thx for your help just wanted some more opinions