Start flushing now. You want to do that when the leaves begin to change color. Use a much larger reservoir next time. The smaller it is the more it shifts. It shifts temperature much more due to less volume, ph will fluctuate more because there is so little water to account for the nutrients left behind, and you run the risk of it drying out if you can not get to it for a day or two. Get a low profile Rubbermaid or something similar for a reservoir. 3-5 gallons per plant is a reasonable guideline for DWC grows. Also 600ppm is not too high at all. I run GH's Maxi series at 1000 or more and have no burn or flavor issues.