I can't tell from those pix, but I would say that if it is the bigger leaves that have yellowed first, look for a nitrogen or magnesium deficiency. Are you using a reverse osmosis filter to filter your water? If so, I would probably say it is a a magnesium deficiency (filters remove most all of the beneficial minerals in the water including magnesium and calcium). Also the spots are NOT too typical of a nitrogen deficiency, so again without better pix, I would again say look into a magnesium deficiency. 1/2 tsp per gallon foliar spray or 2 tsp per gallon soil drench of epsom salts (available at grocery store) should fix the problem if it is a magnesium deficiency. Or you could use Cal-Mag or Mineral Matriz from the grow store, but epsom salts is more affordable. How often are you fertilizing and are you fertilizing at full or 1/2 strength and what are the holes in the sides of the containers for? Otherwise, the leaves look very flat and healthy. Looks like you will have an easy-fix on this problem!