Lowering the pH is a more extensive task. If you simply add a product such as “pH Down” this will not work. Your buffering system will simply very quickly raise the pH back to its original state. You must remove the buffering ions from your tap water so that you may lower the pH. The best way to accomplish this is to purchase a Tap Water Purifier unit. These units filter the water from your faucet using an ion exchange resin. The resulting water is free of the salts and minerals which buffer your water. Aquarium Pharmaceuticals makes a nice compact Tap Water Purifier specifically designed for aquarium use. This is the only reliable method I know of to reduce the buffering ability of your aquarium water and to lower pH. Without purified water, you may be able to lower your pH for a day or two, but without first removing the buffering ions your pH will climb again to natural levels. This fluctuation in pH is much worse than having the wrong pH to begin with. As an additional option, I should mention that many hobbyists use peat to soften their water. By running peat in your power filter, or by placing a layer of peat under your gravel, you will soften your water. This technique can work well, but is more complicated, less predictable, and probably best avoided by the inexperienced hobbyist.