Trouble diagnosing this - any help?

I left a post here a few days ago about some of my lower leaves turning yellow and yellow spots appearing on some of the higher leaves. I thought it was a pH proble at first, but now I'm thinking it could be a magnesium deficiency, or even a nitrogen problem.
There are no more spots on the higher leaves, and the few that were there aren't growing bigger, but the lower leaves are getting more and more yellow. Mainly between the veins (which are still green).
It's my first grow so if you could take a look at the pics and tell me what you think it is and how to best fix Id really appreciate it.
They're skunk, under 600W hps, 17days old, using biogrow every second watering.
pH of the water I'm using 7 (used to be 7.8 until a few days ago).
I'm using some indoor potting soil that my room-mate got us, don't know the specs for it unfortunately.
Any help...?
Im feeling pretty attached to these girls since they're my first, wanna do right by them so they pay it back to me

you're fine, those lower leaves are supposed to yellow and fall off like that. the spots look like you got some water drops on the leaves, which under an HID lamp will cause slight burn spots... those plants look great, good luck.

They look fine to me.

Thanks a million guys
In all the grow guides I've read nobody has said anything about lower leaves turning yellow and dropping off.
That's a load off

100 percent id wait a week then transplant to bigger container. Id use some fox farm ocean forest and give em st8 water for a while...