I grow with rockwool myself,the best time to leach out salts is when you change the reservoir(about every two weeks)as you drain the tank,pour fresh water over the blocks,make sure the water is at room tempeture so there is no shock,repeat this several times,about two or three gallons ought to do,you should be able to use the pump for this too,just divert the flood hose to a sink or toilet or lawn,you might have to use a longer hose for this,never let the pump run dry either cause it will burn out,the pH for the rinsing dosent really matter,and refill the res,add your nutrient then adjust the pH,and use a airstone or bubble wand in the tank always, and when you add back water,pour it in the flood table itself,that'll slow and minimize the buildup,keep the res temp at 68-72fH,a res that is too cold will lockout nutes also,too warm,and you'll get algae or fungul formation,silicon is a great additive,keeps the plants green without adding extra nitrogen,you should be able to find it at your local hydro shop....bonbon