That hydro store is not to be trusted, they just want to make a sale. Most of those flushing agents kill the good bacteria you have going around the roots, so they sell you the stuff to kill it then they sell you the stuff to replace it. Usually those places are the worst places to ask for advise because they have other motives 

My tips burn a little after the flip every grow and if it gets worse I just flush with plain water and hydro guard for a couple of days and then go back to feeding at 1/4 strength then ramp up slowly. Soil may be different though.