If your runoff is still high, you can hand-water the slab with pH 5.5 until you get it down around 6, and then preferably change your res. IF you don't want to do this, just drop your res pH to ~5.6-5.8 and you may notice it rising gradually back up. Continue to adjust it to slightly lower than your target until the drift stops.

You don't need to flush unless there is a problem. But change the res every 1-2 weeks to prevent nutrient imbalances.

Next time, pre-treat your RW by soaking it down to pH 6 and then shaking out excess water before introducing plants. And when you stack some paper, grab yourself an EC meter. These things are REALLY useful to hydro growers. You may not NEED one sicne you are good about res changes, but when the time comes that you want to dial-in a strain perfectly, an EC meter and a log book are your 2 best friends!