to do a flush take straight ph`d water at about 6.5 or 7 ( nutral) and run about 2 to 3 times the amount of soil through it ( eg 1 gallon pot 2 to 3 gallons of water. then let it sit for about an hour or so to drain a bit then add a nute mix of what ever cycle your in ( flower or veg ). then let it dry for a while till the leaves almost droop. wet/ dry cycle is most important i use the weight method instead of the finger in the soil to see if it needs watering, its more accurate for me.
but then again i just realized you may be doing hydro. if so disregard my help on the flush. i dont do hydro i`m an old dirt lump myself it`s easier to deal with.