Not sure what's wrong with this plant

EU624

Member
I've got five female plants, white widow, indoors, aged 8 weeks, going under a 400W Metal Halide lamp, situated about a foot under the bulb inside a medium sized closet, I keep the lights on 18 hours and off for the remaining 6, and I have a 400W HPS bulb I'll switch to when they're ready to start flowering. I have an oscillating fan in the room in addition to vents to keep the air circulating. For the growing medium I'm using Scotts potting soil in one gallon fibreglass pots, with a mix of gravel, sand and beer caps at the bottom for optimal drainage. I adjusted the PH to 7 from 8 using small gradual distributions of citric acid from Hamlin Oranges, and I have a moisture meter on hand for watering, I like to give the plants 1 Cup of water whenever the meter reads 3-4 on its scale of 1-10, 10 being the wettest, a measurement which I'll probably increase as the plants continue to grow; but I add two and a half cups of water to them once every four days. As for Nutrients I'm using Miracle-Gro's '24-8-16' all purpose water soluble plant food, of which I mix in 2 ml into 2 liters of water every 15 days.

Sooooooo, with that out of the way, thus far I have four healthy plants and one that looks like it survived chernobyl. About four weeks ago it started down its peculiar path, as you'll see in the picture I've attached the leaves are all twisted and deformed. A week ago the newest set of leaves on it were relatively normal compared to its recent predecessors, but its gone back to, well, what it looks like in the picture. I've checked into self-diagnoses pages around the net to no avail. So, I guess I have two questions relating to the plants situation. 1. What's causing it? And 2. Will it affect the plants health and or yield?

First picture is the problem plant, the second is one of the normal ones, which, except for a touch of fertilizer burn at the bottom leaves, seems to be as healthy as a horse, or a horse's plant equivalent. (Note what appears to be spots on the bottom leaves on the problem plant is actually just reflections of the camera flash, sorry about that):

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