Hi,
Don't over do it with the nutrients as its not reversible, nor wise. A lot of problems can come with overfeeding. Its the older growth that is affected which is pretty normally as they're most likely the leaves not receiving any light through the canopy. The only time you should worry about yellowing leaves is if its the middle of the plant or new growth, but from what I see its all old growth. You've got nothing to worry about. Do a bit of your own research and you'll see this for yourself. Good luck.