Dark green veins; brown spots... what's up?

I'm 3-4 weeks in from seed... 5 plants, 1 is struggling. 3 started with brown spots starting on the outer corners of the lower most leaves... I checked the pH, and dropped it from 7.8 to 6.0. This seems to have fixed the problem for most of them, except one. This one is growing unbelievably slower than the rest and seems to be sick. It only has 3 sets of leaves, all of which are light green with dark green veins, as well as the first set of leaves (the lowest set) continuously browning. I have a control plant, which is growing perfectly, so I know it's not the amount of nutes or pH... this plant just seems to be struggling, what can I do to help this one? Could this one just be a struggler? My control plant is a mother plant growing in a separate drip system. The others, are in a 4 bucket DWC central reservoir system, and are thus getting all the same nutes and pH. Why is just one plant struggling? All help appreciated, thanks.

Why are you setting at 6.0? Bring it down to like 5.5 if you're drifting a lot.

And if your PH drifts that much you probably have a bacteria problem.

Green veins and brown spots sound like magnesium. Are you feeding magnesium?

It wasn't drifting that much I just didn't take care of the issue at first. I brought it to 6.0-6.5 because I didn't want to shock it coming from 8.0. I'm going to put it down to 5.5 tomorrow. I was thinking either magnesium or potassium deficiency, but why just that one? I'm using Fox Farm's 3 part; so currently Big Bloom and Grow Big, with 310ppm.

Just adjust the ph, it's worse to leave it high. How the hell can you have a "control plant" and then be 3-4 weeks from seed? Doesn't make sense. If from seed they can, and probaby will, be different. Maybe just bad genetics.