Bubble carbon dioxide through the tank. The carbon dioxide will break up and form carbonic acid which will help lower the pH. However, the pH will increase once the carbon dioxide ceases, making this an expensive solution.
Use phosphoric acid buffer. This will bring the pH down to around 6.5, but will increase the phosphate levels. This is problematic because algae thrive in tanks with high phosphate levels.
Add muriatic acid, also known as hydrochloric acid. While this can be a very effective method, it also has the potential to lower the pH drastically; follow the instructions carefully.
Use a tap water filter. These filters remove minerals like sodium, fluoride and potassium by using small resin pellets to absorb the impurities and restore the proper pH level to the water. Tap filter systems can purify about ten gallons an hour.