The only natural way of lowering pH is to add ornaments that release tannins, as tannic acid is the compound that lowers the pH, like bogwood, driftwood, peat moss, almond leaves, alder cones etc. I found my driftwood from the beach and it worked marvelously. I also found the alder cones from a local park and I have yet to try them.
If you want to lower the pH without the use of tannins, there is reverse osmosis water, but it can be pretty expensive. There are also pH-altering chemicals, but I wouldn't recommend them as the pH would just bounce straight back up again after a while. Hope this helps!