You should use Revers Osmosis (RO) water if possable. This is water that has the metals, phosphates, and other impurities removed. This is the water that you should always top off with. Then there is distilled water. (DI) or RO/DI water would be the best where both processes are combinded.
Look in your phone book for water and ask how much they charge for it. Meijer will usually carry RO water.
Thomas