Dwc help, yellowing seedlings

My 4 girls look decent but I noticed some of their bottom leaves yellowing. They have been in dwc buckets for almost two weeks. Ph stays between 5.8-6.5, ppm 300 (120 tap water) some grotek kickstart and a little calmag also some uc roots for sterilization. Water temps at 72f. Hopefully someone can give me some advice as they seems to be growing slow and I still haven’t had any roots drop to the bucket yet. Feeding from the top 15 min on and 30 min off. Thanks here are some pics

I thought I answered this

even 15 seconds at this stage is waaaaaaay too long. I wouldn't do that in full bloom

Heres my 3+ week ld roots from seed

3 SECONDS on ~ 60 minutes off

raise the ppm to 600. after a while raise it some more. you're probably going to use around 1000 ppm max as time goes on.

With hydro you don't need high ppm. During flower I never go above 800 ppm/i.5 EC