Problem with seedling turning yellow.

Need a little info here as this is my first grow. I have two seedlings in soil and one in a small bubbleponic setup. The two in soil are doing fine (a nice organic soil with dolomite already added) but the seedling in the bubbleponic setup seems to be turning yellow. This just started today and this is probably the 5th day out of the ground. I don't know if I am just being paranoid or what, but I took a picture to see. The ph of the water is around 6 and I am not running any nutes yet, just straight water so I don't know if the plant is starving or what. Also, I have had the light on 24/7 and I am wondering if I should put it on 18/6 because I don't notice a lot of growth. Thanks in advance for the help.

...are you let'n them dry out a lil' inbetween waterings?
...they may need a 1/4 strength solution.

Well, the seedling turning yellow can't dry because it is in the bubbleponic setup which is super oxygen water all the time and the roots sit in it. I am just getting roots to reach the reservoir now. The other 2 in soil are doing fine and are a nice green.

...I think it's time to fertilize'em @ 1/4 strength...maybe even lower that pH a little.

I was thinking as much but didn't really know. Thanks for the input and I will commence mixing my recipe for success.

Looks like it needs some micros, magnesium, specifically. When the veins stay green like that, while the rest if the leaf yellow's, it's a good indicator of a mag deficiency. Got any CalMag, or something similar?(assuming the PH is correct)

I didn't even start nutes before this happened. So I am putting it on 1/4 strength to see if it improves.