deficiency, not over feeding. common mistake. There are all these people on the forum saying "I don't feed them anything for the first 3 weeks" or other such bullshit. Newbs love to try to slam nutrients into their plants because they have big errections thinking about all that weed they are going to get s there are many people that caution on over feeding.
For me, with my nutrients I generally follow the following PPM's... (subtract your base tap PPM).
clones until 1-2" roots 100-200
rooted clones 400 (but easy on the nitrogen)
4-5 nodes.. 600-800 (800 may be on the highside).
from 5 nodes to 6 weeks 800-1200 (I have never seen a vegging plant that wanted higher than 1200.
FLUSH
Flowering
I bounce around a bit between 1200-1500 but I hardly ever ever go over 1500.
If I do, then I take a little water out of the rez and add fresh water. I don't add back nutrients to the rez until the PPM gets below 800. But in flower, you really have to let the plant guide you in terms of what they want and when. Flowering is intense for a plant and they want a certain element, they typically want ALLOT of it. For this reason, I employ a routine foliar treatment with kelp and LIGHT flower nutrients/cal mag.
Hope this helps.