Help.....please

Im a noob when it comes to this, whats the most likely cause of this happening to the leaves. I dont know if they are indicas or stativas, i just know the name it was given "purple princess". It was from seed. They are aprox. 2 months old, growing indoors with a sunleaves pioneer VI 6 x 54w relamp with T5 HO 54w fluorescent type lamp 120v 3A 60Hz 324w. The bulbs are vitalume plus Grow bulbs. They are in 5 gallon buckets using Scott's moisture contol soil. I used root and grow for their first few waterings to establish the roots in the soil after that i have not gave them nutrients at all since then. I water them every 4 to 5 days depending on the soil moisture. I have it at a constant temperature of 75 degrees f. With fans going for proper ventilation. The plants are on a 20/4 hr light cycle with a timer so its constant. Everything i do is the same for all my plants and only one plant is having issues. Ive scoured the internet for a solution to my problem but came up empty handed. Ohh and since i live near mountains i get the luxury of using natural spring water which i know is safe because I drink it.

I forgot something i use natural pesticides also (a combination of steinernema and heterohabditis nematodes) about 500 per bucket

Looks to me like you heat stressed it off the lights. Was that leaf close to any of your lights ?

Other than that:
I would say it is a phosphorous issue. Phosphorous is vital for a plant especially during the bloom stage. It is the most important for blooming.

That's what i thought at first, so I raised my lamps to about 5ft above them; I still have that problem. What gets me is that its only the one plant.