PH Fluctuations and/or Nutrient Issues?

With a dramatic drop in pH, a res change is always a good idea. If the EC rises as pH falls, then you're feeding too much. Here's where it gets interesting. If your EC remains stable while pH falls, then raise your EC if its lower than 1.0 and lower EC if it's over 1.4. This is because of a possible leeching effect from your plant into the res. In any case, ALWAYS do a res change if pH drops dramatically. It's a pain, especially if it's only been a few days since the previous res change, but believe me it's worth it.