Try looking through this. I'm gonna go with my gut on this one so don't take this as "professional" as I may be wrong. Doubtful it's nitrogen, your plant looks nice and dark green, nitrogen deficiency appears as a yellowing of the plant, it cant be nitrogen toxicity as there doesn't appear to be a claw symptom on leaved and while your leaves are dark I think that's genetics. Most of the immobile nutrients show signs at the top first and leaves typically get lighter in color so I don't think it's that (your leaves look like they're getting darker from the outside to the inside then withering from the tips, you said it also starts from the bottom so prob a mobile element). My money is honestly on a phosphorous deficiency. Definitely one of the mobile elements. I'd just flush it with like 1/4 strength nutrient water to flush out any imbalances and then look for the problem to dissipate. If you're controlling temperature, pH, PPM air flow and all of the environmental stuff then it has to be something with the nutrients or pests. Check this out:
http://www.growweedeasy.com/phosphorus-deficiency-cannabis.