OK guys, let's see what happens after lowering PO4. As Defdac has already said it seems it helps. Anyway, now i'm going towards low levels (at least PO4). A year ago i had 2..5 ppm NO3 and not detectable PO4 (say 0.1 ppm max) and plants grew like crazy; no burned, stunted tips, no deformed leaves. At that time i didn't know the word "nutrient deficiency"