Leaves keep turning yellow-please help!

I just started growing using ebb and grow system, with clay pebbles as grow medium (recommended by the sales person). I plant some Baba Kush and King Kush recommended by the clone store sales person. I started with 5.8-6.0 ph, and 700 ppm nutrient. Leaves kept turning partially yellow, mostly in the middle of the leaves, and some of them stay the same, but some get worse and die off. Someone suggested I try to increase water frequency as clay pebbles don't hold water as long as other mediums, and raise ph to 6.0-6.2, and raise nutrient to 800-1000. Which I did and no significant improvement observed. I use the commonly recommended NPK ratio for veggie, with all micro and trace minerals included.

Attached are 2 pictures showing a partial yellow leaf plant, and one with some leaves close to dying. Any suggestions on how to fix this problem please help! Thanks very much.

Attachment 295572Attachment 295573

Here's another better picture showing yellow leaves. Attachment 295604Attachment 295605Attachment 295606

You seem to love your plants and seem to be doing much of the right things. Other than testing ph/acids I try the keep it simple cause im stupid method. Check some things out... does too much magnesium, calcium and other minerals have neg effects, answer yuuup. Hows your soil with other nutrients? What if any of these nutrients are in the clay you use? Are you watering with well water? whats in that? try distilled water through your next 4 feeding cycles and see whats up, should be feeding every other water. various water sources have clorines and metals that are not good for plants, its ok to germinate with but that's about it. some strains are prone to various colorations and by the pics im not seein abnormal colorations. actually had leaves die? Color on your plants are lookin tasty hang in there...

p.s. transplant one, sieve the soil lookin for bugs and or eggs, mites etc.