One can raise or lower pH by adding chemicals. Because of buffering, however, the process is difficult to get right. Increasing or decreasing the pH (in a stable way) actually involves changing the KH. The most common approach is to add a buffer (in the previous section) whose equilibrium holds the pH at the desired value......
Muriatic (hydrochloric) acid can be used to reduce pH. Note that the exact quantity needed depends on the water's buffering capacity. In effect, you add enough acid to use up all the buffering capacity. Once this has been done, decreasing the pH is easy. However, it should be noted that the resultant lower-pH water has much less KH buffering than it did before, making it more susceptible to pH swings when (for instance) nitrate levels rise. Warning: It goes without saying that acids are VERY dangerous! Do not use this approach unless you know what you are doing, and you should treat the water BEFORE adding it to the aquarium.
Products such as ``pH-Down'' are often based on a phosphoric acid buffer. Phosphoric acid tends to keep the pH at roughly 6.5, depending on how much you use. Unfortunately, use of phosphoric acid has the BIG side effect of raising the phosphate level in your tank, stimulating algae growth. It is difficult to control algae growth in a tank with elevated phosphate levels. The only advantage over hydrochloric acid is that pH will be somewhat better buffered at its lower value.
Peat moss softens water and reduces its hardness (GH). The most effective way to soften water via peat is to aerate water for 1-2 weeks in a bucket containing peat moss. For example, get a (plastic) bucket of the appropriate size. Then, get a large quantity of peat (a gallon or more), boil it (so that it sinks), stuff it in a pillow case, and place it in the water bucket. Use an air pump to aerate it. In 1-2 weeks, the water will be softer and more acidic. Use this aged water when making partial water changes on your tank.