I would say over fert or ph problem try backing the nutes off a bit also how close to the light are they? it definately look like some kind of burn with the way the leave edges are twisting and curling up.
Good point it could definately be the roots are your roots nice and white or are they brown and slimey? A good way to tell is just kind of tug a lil bit and if it's root rot they will just fall of in your hands healthy roots will not. http://www.technaflora.com/products.php?product=19
that is a...
Its hard to say these pictures are a lil week, are they burning anywhere? Are the leaves curling under? Any noticeable spots or necrosis? What kind of sytem is this? Cal mag seems to make some since but you added it already any noticeable change? Is the yellow still progressing after you added...
I'm thinking it looks more like not enough trace elements since they effect your younger growth. Nute burn would be burning the leaves, making them curl under and they would be a darker green. Also like irish farmer mentioned having that many plants in that little of a pot is bad news, they will...
I'm thinking over fert or too much watering. I would cut foliar feedings and see if that does the trick. Also what is humidity at? Probably over 50% huh? Try looking up moisture stress and see if anything lines up. Remember anything you do will take a couple days to show improvement or worsen...
I havent grown soil but I know guys here are gonna need specifics like ph, ppm, lighting and distance from light, soil make-up things like that the more info you can give the better and more precise answer you'll get.
I was asking what your previous ppm was to see if there was a major jump between then and now but I guess you have always ran it at 1200 so must not be nute burn, even though i think if you were to run ppm's at 1200 in early veg it would be logical to think you would burn a plant but you must...
What was your ppm on your last nute mix? Plants look nice and green and dont seem to be deficient I would say nute burn from looking at this but more info is needed. Also if your 3rd week into flower where are the flowers? when your lights are off you need to make sure there are 0 light leeks...
What kind of system is it? I'm assuming dwc from your explanation, nutes can have an effect on root color so if it looks like your nutes I would just stick with it, but if the water temp is over 70f than your asking for the rot I would definately lower the water temp to maintain below 70 even...