Whats the PH of your growing medium? Also, you mentioned they are 8 weeks old. Have you repotted them? It looks like it could be a possible nute deficiency. I'm not sure if your able to do so, but try and check the PPM of your runoff water coming out the drain holes of the pots, that will give you an idea of how much nutes or salts are in the growing medium. Once you get the PPM, you will be able to determine if there is a salt buildup or deficiency. Hope this helps you. Happy growing