Your biggest, oldest sun leaves changing color doesn't always mean something is wrong, you have a lot of healthy looking foliage in those pictures. No, I wouldn't be happy with that spotting either, but drastic changes aren't my thing. Personally, I wouldn't go crushing up epsom salts just yet, if you have well balanced nutrients to begin with then you don't always have to make corrections on that end. I would, however, address your PH. If you are using city water and you aren't already setting your buckets out for a couple of days then start, it will evaporate out the harmful chlorine, if you can pick up an air pump and air stone from wherever you got the aquarium ph meter, even better. Whether you are due for nutrients or straight water, PH your final mixture to 6.8. Tap water alone around here requires very little PH down, while a nutrient mixture typically requires comparatively more PH up. Always be precise, if you are not then you can't target problems as easily or accurately.
Regarding the flush. Every other watering should be with clean, PH'd water. A medium like promix HP is good because it drains readily, preventing root rot, and allows the plant to eat off of just what you are putting in the water bucket (and your air and sunlight of course). Also, by having consistently PH'd feedings I've never tested my runoff, but never had signs of PH issues either in multiple grows. By watering with clean water until the pot runs out the bottom (I recommend smart pots for a host of reasons) you are not only cleaning buildups from the roots and allowing clean uptake of the nutrients you give, but also running a clean base through the plant: this is partially why I recommend letting your soil go pretty dry before watering (Water, dry, nutrients, dry, water, dry, nutrients, dry) If your leaves aren't sagging then your soil is not too dry, my girls are top heavy at their driest. You want everything to be taken up into the plant and then everything replaced with your next application, whether it be nutrients or water. With consistent alternating water applications I do not personally see a reason to flush anytime other than two or three waterings before harvest. At that time, yes a thorough flush where the pot runs through for an extended period of time is advisable (as well as really letting them dry out between, like saggy leaves and all. Some think that this amps resin production in an effort to protect against the dry conditions).
I hope some of this input helps, there are many worse looking plants on the site, talk her through it and she'll level out.
