Unless you have a specific reason to adjust your ph I would not tinker with it. That being said. A natural way to lower ph is to use peat or drift wood which releases tannin which lowers ph. (this will also stain your water like tea) this is good for Amazonian setups ie tetras etc. another thing you can try is to place a used decaf teabag in your tank or filter. This is actually good because it also releases a small amount of nutrients for your fish and plants. Both tea and drift wood have anti bacterial properies so plants of good. These are in general best for creating black water conditions specific to a certain biotope however so if you don't have a real reason, I wouldn't worry about your current ph.
