*got clones NOV 16 2017* they arrived on a -10 degrees celsius day with tons of snow lol, using local nutrients, started with four plants three in dwc and one in dirt because it was my first cannabis grow and i was nervous about dwc, Here is the nutrients i used: https://imgur.com/a/9Nouj
i concur with your diagnosis -- ph fluctuation
if it continues and spreads watch the pattern sometimes light burn starts between the ribs of the leaf like that as yellow and spreads
especially if it gets suspiciously even patterning -- ph spots are usually irregular spots
@DojoGeejer, never used it for that purpose but i am a big fan of potassium silicate in coir, makes much stronger plants
it will definitely make the roots of any of your plants less susceptible to shock from any source so it should work to help against ph shock i suppose?