At least it the beginning, the tanks need for supplimentation will be minimal. The best place I recommend you start is with the water you use for water changes. More commonly, regular weekly water changes will do what's needed to maintain chemistry/trace elemnts. Correcting the chemistry if needed in the new SW
also gives you a "save zone" to experiment with chemical aditions like the baking soda Brenden mentioned as well as other two part liquids or stand alone chloride/buffer products.
Once you get the appropriate test kits, test the tank and the new SW
used for water changes. If the new SW
and the tank are relatively the same but low, it will be far easier to tweek the SW
mix and less chances for errors or disasters. At least in the interim while you learn the basics.