Play the devil's advocate and ask them. carefully argue with them since they will divulge more information if their idea is challenged. Also find out if it is rock salt Nacl, or ice melt which could be any number of things that folks use in planted tanks. Eother way there reasoning is probably due to a half truth or a myth, but LFS's are just as succeptable to myth as anyone else. The friend I mentioned in the original post worked for an LFS for 5 years, that is where he learned that all freshwater fish require salt. He learned a few other things at that LFS, but still knows nothing of the nitrogen cycle. Before his employment there, his fish were much better off.
In the FWIW Department, the title of this thread is "Long post: Information, and how to asses it". I think you mean assess, which is to determine the value, significance, or extent of. As written, you have plural posteriors.