I think Dagweed might have it; from the other pics of your grow, this looks to be an outside green house? Where pests are more likely to be an issue. In the middle pic, look at the top leaf in the bottom right of the image - the tiny circles look like bite marks, and tend not to happen with nutrient burn. The rest of the damage to the plant is probably related to the damage from spider mites. They are *really* tiny.