Epsom salt mixes are normally used if there is either a calcium or a magnesium deficiency in your plants. Yellowing of lower foliage is normal over the course of time. Fan leaves typically can get purple and even strains such as my blueberry naturally have purple stems. That is good you are diluting your fox farm nutes because if you use the recommended dosage on their label it does not apply to all marijuana plants because some strains hunger for it while others do not tolerate it. The question would be what stage is your plant in? Vegetative or Flowering. The next question would be at what week is your plant in. If you are way off on the fox feeding schedule then you would be introducing nutrients in the wrong sequence, meaning you could be giving your plant the wrong nutrients in the wrong phase of development.