[WombatJr]

I'm a new grower at about 5 weeks into veg from seed. I'm growing in soil-less mix "Sunshine Mix" growing in 3.5 gallon containers. I have 400W HPS lights (3) over a crop of 26 plants (1/3 Blue Apollo, 1/3 Trainwreck, and 1/3 Northern Lights).
I'm watering with tap water mixed with Botanicare PureBlend Pro Grow organic plant food (3-1.5-4) at 1tbsp per gal strength.
Plants have been growing great up until about a week ago when I started seeing some lower and middle leaves on some of the plans turning yellow, brown and drying up.
I've noticed that my PH might be a bit high.. it's between 7.0 and 7.5. I've got some PH down to try to correct that with the next watering. Could that be all that's wrong? Or are my nutrients messed up?

I've attached a couple of pics from some of the plants. Any ideas?

Thanks in advance for your insight.

WombatJr.