Once inside the plant, some nutrients can be mobilized to support new growing tissues, while others are fixed, or immobile in older plant tissues. This fact helps us to diagnose some plant nutrient deficiencies. For example, if a plant is deficient in an immobile nutrient, then deficiency symptoms (yellowing/chlorosis) occurs in the new growth, since the older leaves/tissues "hold" on to the immobile nutrients. In contrast, deficiencies of mobile nutrients typically occur in the older leaves (like yours is doing), since the mobile nutrients move from the old leaves to the new leaves.
MOBILE: Nitrogen, Phosphorus, Potassium, Magnesium, and Sulfur.
IMMOBILE: Calcium, Iron, Manganese, Zinc, Copper, Boron, and Molybdenum.
I said all of that to say this....Check your nutes and see if they contain Micronutrients such as Calcium and Magnesium. If your nutrients contain Calcium and Magnesium then your pH will probably have been too low at some point since these nutes are taken up at a higher pH level. Most people equate "pale" green leaves with low nitrogen, but it is most likely a Magnesium defficiency. If raising your pH slightly doesn't help (after checking your nute content), then flush with neutral water and get some epsom salts in there.
Also, how much runoff do you have when you water? Could be a build up of salts in your soil. A good flush will help this. Also, do not OVER water your plant(which it sounds like you are not). A whole week without watering is probably a little too long in between though. In soil it will take some time to fix your problem, since it took some time to get it. Be patient and she'll pull through.
Since you are using MG (which typically contains plenty of Nitrogen, which is available at a wider range of pH levels), I would be willing to bet (as long as you have good drainage, to rule out some sort of root rot) that what you are seeing is a Magnesium deficiency.
And if you do not have good drainage then you could not only have a Magnesium deficency, but you could also be getting root rot. What you need to do in that case is mix some hydrogen peroxide in just water (about a tblsp per gallon of water) to kill the rot. Like I say, if drainage is good then you definitely need some Magnesium in there. Good luck.
