OK. So here's an analogy for the discussion you had with the pet store person.
You: I am really hungry
Clueless person: Oh, you should try playing tennis!
You: This has nothing at all to do with my problem. Also I am pretty sure that playing tennis will just make me more hungry and make the problem worse.
Salt has absolutely nothing to do with lowering pH, lowering gH, etc etc. This is probably the worst advice I've ever heard from a pet store employee and I've heard a lot of bad advice. Salt increases the dissolved solids in your water (makes it harder basically, just not in a way that your gH test can see), and in fact will even raise the pH in infinitesimal amount. It basically does exactly the opposite of what you were advised. Do not ever speak to that pet store employee ever again. If you see her, run the other way.
How hard is hard when you say your water is hard? What is your gH reading?
There are fish that are ok with a high hardness and alkalinity. You can either pick fish that suit your hardness, pick fish that are middle ground that you can acclimate to your hardness, or cut with RODI water. There is not a true way to reduce your hardness and pH without adding "empty" RODI water.