I don't grow hydro, but that looks like trouble. Your roots are quite dark as well possibly signaling some root rot, but like I said, I don't know jack about hydro.
I'm growing a Purple OG Kush in soil now. You can check it out on the link in my signature. Seems like she eats the nutes up real quick. At a minimum of 2 of 3 waterings I give her a full serving of nutes and she keeps coming back for more. Seems like around week 3 of flowering things started yellowing up (normal for flowering), but I'm going to hit her with a little extra N to slow it down today and see what happens. I tied down her top in wk 1 on flowering (she was 3.5 ft tall) and it has given me 10+ tops now. I don't think she's going to be a light producer like you said, but I might have just gotten a nice pheno. She has also been very resistant to heat (had a few issues in the beginning, others suffered she pushed through without noticing). Also seems like she's a heavy rooter. When I harvest I bet the 5 gal bucket is nothing but root!