I wouldn't spray my leaves with nutes/additives either. nutes burn leaves generally...In hydro you don't need any faster application than adding to the rez. And I would spray them down with a ph'd water solution.
Just FYI... I have taught some things to my local hydro store guys...In fact, I have surprised them with some of our secrets.
I am not calling your hydrostore guy wrong, but just because someone runs a hydro store doesn't mean they are experts...unfortunately. Remember, anyone can grow hydro if they have all the toys. IMO peace

p.s. You have lost the affected leaves...you have to look for healthy new growth...that is key. from now on, It is the new growth you must monitor...goodluck