re-calibrating

So the plants have been in the hydroponic ebb and flow system for a while, and seemed to be thriving. Then they started to turn yellowish. I moved the light up a bit, as I was concerned they might be getting 'burned' by the light. Finally called the store, and was asked what the pH was...told to check the pH every time I added nutrient. I had added nutrient last weekend. It checked a bit lower ppm than recommended. Yellow sounds like its lacking in nitrogen. Check the pH. When I checked the pH, it was 7. Recommendation is to be around 5.5. Got the pH up/down stuff. Added about an ounce and it came out to pH 5 to 6. right on. shifted the timer so the water would flow and the plants would get a dose of good juice. Now patience again to wait for the greening up of the plants.