"Salt" is a general term use to describe the product of an acid-base reaction. There are literally thousands of salts. Some of them will effect the pH and others will not. We are just going over this stuff in chemistry and the effects of salts on pH. The salt that you specified (sodium chloride/NaCl) will have absolutely no effect on the pH of the water. Salts form ions when placed in water and the ions that WILL effect the pH are salts that are the products of weak acids/bases reacting. Salts composed of the following ions in their entirety (only those ions formed from the compound) will have no effect on the pH: Na+, K+, LI+, Ca+2, Ba+2, Sr+2, NO3-, ClO4-, Cl-, Br-, I-, SO4-2. Example, you can have CaCl2, Ba(NO3)2, and KI (all salts) in your tank and your pH will remain the same. If you have other salts that contain ammonium (NH4+) for example in a salt, say NH4Cl or NH4NO3, only the NH4+ will have an effect on the Ph of the water. I could tell you how much it would change in just water if you told me the amount put in but buffers in the tank would make the calculations off. They would lessen the impact of the pH change.