Veins in Leaves Turning Purple

I'm currently running a bubbleponics system with a 400w mh. I am growing hindu kush.
Current PH is 5.6-5.9 and my ppm is about 630.
The seed sprouted on 5/12/11 and the plant is showing discoloration in the veins on the leaves.

I have a picture but it is hard to see but it's the best one I was able to take.



I've done some searches and the only thing I saw with people having the same problem was that the temperature in the grow room was too low but my room maintains a constant 85-88ºF.
My best guess is a nitrogen deficiency?
Does anyone care to offer an explanation? Any help is appreciated

Also I should note that the bottom of the leaves are almost completely purple

pics suck.. no offence. Can't see anything.

Usually purple is from cold (or strain) If your not expecting purple, chances are its getting too cold at night when the lights are off.


Just my opinion

Im running the lights 24/0 so there is no chance its getting cold. And the water temp is usually around 65-75ºF