Newbie first grow should I start nuting?

Ok plant seems to have been stuck for couple days. Can I start nuting with like 1/4 strength? Leaves have been yellowing and now are pulling all green from starter leaves. I read that means nitrogen deficiency? Some one give their opinion. It's been a slow grow for these guys for some reason. they seem to grow in spurts.. Oh and also on the bigger one the leaves are starting to curl. I put water in the pan when it dries out and spray the rockwool probably once a day to keep it moist. should he be getting more water? 400w Mh bulb about 8 inches above for 20 hours a day.

they look a little yellower than in the pictures.

i don't know if it's a nitrogen deficiency problem but you definetly can start feeding.

I've never grown in water, but in soil you can start them at about 3 wks of age.

k cool yeah ill start him on a low dose thanks yeah i'd say would probably be about 3 weeks old. Thanks. I'm just paranoid because i killed a few seeds on accident by mildly nuting the rockwool the first time, lesson learned.

Get those cubes out of that standing water. That is not a good idea for that medium to be saturated... don't squeeze them, but let water run through them, then the bottom should be out of water.