I read two different ways to do a flush in ebb and flow, I am leaning towards the second one as my ro system pumps out 2 gallons an hour and it would take twenty hours for 40 gallons.
1. Drain the system, and refill with straight water at the proper PH.
Run thru your flood/drain schedule for a day, then drain your resevior again, and refill with proper nutrient and Ph.
2. Disconnect rez line, re connect so it will flow to floor drain sink etc. Hand water from the top down each plant and let gravity do the work. Simple, but a bit of work.
I'm kind of thinking I might drain my res and disconnect my res line and flush by hand watering then start out with new water.