How to lower my PH in my freshwater tank?

Ok, I have had a 5 gallon tank that been standing for 2 weeks. It has a filter and a thermometer also. There are no current fish in it but I plan on keeping a male Betta and a few other fish. Currently my tank ph level is 8-8.5 But Betta's need it at 6.5-7 Should I lower it with a Ph down or leave it as it is? Please help! I am new to keeping fishies :)