DWC Plant problems, With PICS.. What am I doing wrong??

Ok so this is my 2nd or 3rd try at DWC. Ive got Hempy and soil down, but for some reason I just run into catastrophic problems in DWC. Anyways Plants are clones, about 3 weeks old, PPM is about 500, starting PH is 5.8 and it climbs to around 6.3-6.5 by the time a water change is done (weekly) I know they are not taking much nutes because my PPM climbes to around 8-900 with about 1/3 of the water gone by the time a change resivoirs. Growing in 5 gallon black buckets, roots are very white and clean. I run some H2o and some root zone conditioner on each change. Using GH floro nova nutes. I am also using RO water, and I add around 5ml per gallon of Cal Mag.

Everything seems to be perfect, but the leaves are getting black spots on them, and turning super crispy and falling off. Also It just seems like they are not taking up any N, as the leaves were always very light green compared to my soil and hempy plants.

Can anyone tell from this picture what is going on?? I just did a res change today with half the nutes, and no H2O. I appreciate the help!!

Well, looks like it may have been a mag. def. I put 10ml Per gallon of cal mag in each bucket yesterday as a test to see if It would help, and already I can feel and see the difference in the leaves. We will see in a couple of days if its really resolved or not. I have been trying to grow this BF Violator Kush, and this AK 47 for a while now, these were clones that came off the last dwc grow that didnt go to well. Hopefully I can atleast recover these enough to take a couple clones.