My first grow - advice please and pics

Ok guys I have been smoking for about 6 years now and since moving up to college the prices have just about tripled from back home. after wasting a lot of money I have decided to try growing my own. I managed to get eight 400W Metal Halide grow lights from the local community college FREE!:D:D (they were throwing them away!!) Heres my setup:

2x 400W MH lights
7x3.5 ft room
2 cooling fans, 1 exhaust
cool mist humidifier

the temps are ranging 62-69 night to 73-84 day
humidity is about 30%
18/6 lighting for now

I got 6 plants from a friend with a crowded room and they are about 1-2 months in, I'm not quite sure.We transplanted them from small pots to 3 gallon buckets/pots, and mixed organic soil bought from the nursery with some perlite for the dirt. 3 of the plants a runts and have bad yellowing leaves, anybody know what causes this?
I'll be testing the pH and NPK levels in a day or two and that might give some insight on the problem.
Does anybody have any advice/tips/secrets that I should know about before its too late? any help is much appreciated
Thanks

Oops just checked timer, lights have been on 16-8 for 5 days, switching to 20-4 for now does that sound right?

18/6 is good

you could do 24

18/6 lets the plants rest and saves electric.

i go 20/4 with mine...give the plants a little rest time as well as my lights. dude you got real lucky with the lights you got free...thats awesome...good hid lights will be the most expencive thing you have to purchase so you did really good there....from what i can see your off to a great start....u didnt have to rasie seedlings....and there age will bring u a harvest in 2 months or so....when those plants are 2 months old go 12 12 man and have a happy harvest....you got 8 of those mh lights ???? only using 2 what do u plant to do with the other 6...heres some pix of my grow....lil over 30 days old vegg...30 more days to go and then ill flower for another 60 days....enjoy and keep a watch out on this board for PPP journal updates !!! here we have N L and Afghani in this journal ....first 3 are NL ...last 2 are afghani

If those were mine, I would top the two on the right and the 2 in the rear.

I would tie over and down the two up front.

Those are some pretty sparse plants, and i would agree with Harddon, but also i noticed the soil looks really dry, how often do u water? Yellowing could be caused by nutrient deficiencies or excess, so it's probably one of the two. Have u been using any ferts?

nice looking plant cept the yellowish ones. if your not usin ferts it may be a part of the prob...

good luck man!

Ok moved lighting to 20/4 and raised plants 1 ft they're now about 12-16 inches away. I'll lower them when they shoot up a couple inches, trying to get them to bush out a little and get more dense.pH is in the 6-6.5 range so no probs there. I still have to test nuets tho. any recomendations on fertilizer? i know i want something with nitrogen and phosporous for leaves and roots. not sure what tho
thanks