Brown spots and yellowing

You actually have a magnesium deficiency. Often with this deficiency, magnesium is present, but the roots cannot absorb the magnesium properly due to an improper ph. Therefore it is very important to maintain the correct ph and make sure your ph does not get too low/acidic in order to avoid a magnesium deficiency. I would try bringing my ph up to between 6.0-6.5 that way the magnesium can be absorbed by the plant.