New Plant Dying?

I got a new set of clones on Friday with 3 different strains. Two of them seem to be doing fine but the third strain is yellowing a bit. The picture of the plant in the first picture is the worst affected and I can't seem to figure out why.

The second two pictures are of minor damaged plants of a different strain that came like that. I'm hoping it would clear up naturally but I'm not sure if there is anything I can do?

I'm running them on DWC with a pH hovering over 6. I had the nutes at about 400 ppm. 600 w MH on 18/6 about 2 feet above them. Temps are staying between 75 with the light on and 70 when it's not.

You never know when you get clones elsewhere besides from your own mother plant right?

The first pic looks like its not getting enough air to the roots. Whats your air stone situation like?

It's a recirculating DWC with an airstone for every 4 plants. It also falls into a bucket before recirculating which helps get more oxygen into it. Do I need more oxygen than that?

What do you mean the ph is 'hovering' over 6. Get it to 5.8 and see how they are in a week.