You shouldn't let the water drop more than 5 degrees in general for fish. FH particularly like water in the high 70s/low 80s, so for your tank, it shouldn't drop below 77F while doing a water change. You need to mix in hot water tap at the same time while filling.
Just don't go adding expensive aquarium salt. I say expensive because you can get the same results from water heater salt (sodium based), pool salt, table salt, pickling salt, and any Sodium Chloride product that is cheaper than API aquarium salt. Working at Krogers, you should be able to get pickling salt real cheap.