yellow leaves

need help !!! got yellow leaves that are curling whats wrong ? & is this harmful to the plant ?

U might have a nute burn problem or deficiency. Pics?

Have you fed it?man it could be so many things.Little more background and a pic would do wonders.

took out of grow box for picture purpose but the plant only gets water is the curling normal ?

Forgot to add that the plants are only 3 weeks old any helpful ideas are much appreciated

I think you might have overwatered them. Do you let them dry out between waterings?

What's normal is to search for "leaves yellowing". You'll find 300000000000000 posts on this same subject with pictures just like yours. Spend some time searching first, it helps me keep my post count down.