The sweet spot is when both your EC and pH stay the same... VERY hard to achieve! There is a good chart to follow to figure out what's happening with those fluctuations and what to do about them.... look up EC and PH Fluctuations in Hydroponics - it's on a site called Percy's Grow Room... there are TWO charts - one for when your plant is drinking water and one for when your plant is NOT drinking/taking up water.... (I'd give you the link but GD has gotten a bit touchy about that and generally it results in being banned from commenting/texting... stupid)
Good luck!