Just use clean fresh water for the last 1-2 weeks. Clean your rez by hand inbetween feedings and theres no need to "flush" your DWC. Maybe a soil grow but not DWC. infact, I just cut off a taster branch on my NLx Skunk last week when the plant was 7 weeks, no flush at all and it tasted really good. 1 week into the flush ALL leaves are completely bright yellow. Flush = DONE