I've found with DWC, keep the PPMs on the low side, don't need to go over 900 PPM, even at the peak in flowering. You'll avoid a lot of issues if you keep the feeding light. It's very easy to over do it. If your using R/O water, get yourself some cal/mag. I have a friend
that uses Botanicare's, 3-5ml per gallon, depending on flower time. Change the res once a week. Measure the PPM and PH before topping off. If the ppms are the same and the water level is down, your in the sweet spot. Top off and add nutes to get back to that number. If PPMs are down and water is down, they are "eating" more than what you are giving them, add more nutes gradually, and don't boost PPM's more than 50-100 points at a time. If the PPM's are higher than originally, they are drinking more water than taking in nutes. Top off with only water and lower the nute strength 50-100 points until they start feeding again. As far as PH swings, you'll notice that when your in the sweet spot for PPMs, the plant is happy and PH doesn't swing much, maybe a point in either direction. I found when over feeding, your messing up the system and the PH will swing way over 7 in no time, basically overnight. For flushing, it only takes a few days at a PPM of 200-300 for the plant to really start yellowing, so continue to feed till the very end, then a 4-5 day flush. After 3 cycles through, and reading a ton of misinformation, this is what has worked for me, trial and error. Best of luck