Kind of hard to see the color with the hps on, but they look really dark green. Too much N can cause that leaf curl.

Plus with a canopy temp that low I don't think the CO2 is really helping much...
http://boards.cannabis.com/advanced-...ent-guide.html

ph of 6 is pretty low, assuming you are in soil? Same thing goes for watering every other day, maybe a bit too frequent for soil (but I don't really know much about your environment).

Different strains will have different needs (even the same strain in seed form, if they aren't stable genetics). Gotta figure out what each one likes and treat them accordingly. Some drink more water and like more nutes, others don't.

I know you touched on some of the information already, but filling out the copy-and-paste troubleshooting form here will help out a lot. The more detailed information, the better. :thumbsup: