or closest to your lighting perhaps? Over time my plants nearest the 1kw will take on some interesting lightburn patterns.

I think they ARE underfed which is why jash needs to A)replace his nutes with completely fresh nutes at LEAST once a week.

and B) Loc Nar is absolutely right about the ec.....it's seems fairly stable to you because you are only reading the TOTAL NUMBER......and if your plant eats one nute and not the other you simply are not going to know by checking e.c. Keeping your nute ratios balanced is pretty critical in hydro.....and the only way to do it properly is change out to fresh nutes every seven days. I personally do mine every 3-4 days.

by flushing I mean running pure RO or distilled water "(ph correct) through your system for about 24 hours. You can use a product like Clearex or Flora Kleen as well but the water will work too. Consider adding a good root product to your regimen......you'll thank me later.

I personally think your issues are soil/fert/salt buildup in your medium and poor general maintenance in general alongvwith the plants being underfed. fix those issues and I bet you'll grow some fine ganja.