Flushing should be done ONLY when the plants have been poluted in whatever manner and should not be a routine thing.
For a clean product at harvest, all you need to do is stop all additives but water 2-3 weeks before harvest and I also like to give them only as much water as needed to keep them alive while they "self-clean". Nothing but the cleanest, PH corrected water you can find and that alone will be a proper "flush" and a method much safer than possibly drowning your roots needlessly and causing an early harvest.