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
latewood Reviewed by latewood on . Nute burn? Mg? K? - Getting really bad. Hey everyone. First time hydro grower. I'm been having some problems with my plants and haven't found the problem yet. Now it looks like they are going to bite the big one. The symptoms started with some brown/copper spots on the lower leaves. They started near the middle of the leaf, in between the veins as a single brown/copper spot. It then quickly progresses into larger brown/copper areas. Eventually taking over the entire leaf. It started on the lower leaves and has been makeing its Rating: 5