Pics would help. Yellow spots can be caused by just about anything, pH, nute deficiency, nute burn, bugs. Rule out the most obvious and easiest things first. Good luck my friend.
1st thing that i would do is check the under side of the leaves to make sure that it is not spidermites.Look for small black dot's.If there is no sign of any then it will be a ph problem just make sure that if you are makeing big batches up test every feed because it will alter while stood even with a air stone in.I would also get into a habbit of flushing the plants more than just the once.I flush a couple of times during veg and once every week during flowering.