In many places, tap water is hard and basic. Some fish species originate in soft, acidic water and cannot survive outside these conditions. If you need to lower the pH of your aquarium, you have several options. While pet shops sell extracts and chemicals to lower the pH, more passive methods like peat or driftwood slowly alter the pH, and tend to not produced dangerous sudden changes.
Blackwater From Wood
Many fish live in tannin-stained rain forest waters. Tannins discolor the water to the point that looks like cola or coffee. These waters are called blackwater, and have a low, acidic pH from the tannins. You can re-create these conditions with certain types of driftwood. Malaysian driftwood and mopani wood can both leach tannins into aquarium water, lowering pH and coloring the water. Always procure your driftwood from pet shops, since wood can absorb and leach toxic chemicals. This method works well for long-term maintenance of aquarium pH and tends to work best when combined with other methods.
Bags of Peat
You can use peat to lower the pH of aquarium water. Use untreated peat, not peat with fungicides and other chemical additives such as many hardware stores sell. Just put the peat in mesh bags or old nylons. You can leave these bags in filter boxes or sumps. Like driftwood, they slowly release tannins. Additionally, peat performs chelation, the absorption of minerals. This helps lower the pH even more by removing buffers from the water. This method tends to produce fast, dramatic pH swings; you should use it only when first setting up an aquarium, since it can shock fish when you use this method on existing aquariums.
Diluting to Remove Buffers
You can slowly dilute aquarium water with very pure water. Minerals like calcium buoy the pH, or buffers it. This makes the water resist changes in pH. However, you can purify water through processes like distillation or reverse osmosis. These processes strip calcium and other minerals from water. Water purified this way is soft and more easily changes pH. You can dilute aquarium water with reverse-osmosis water. This makes it easier to lower the pH. This method works best when performing maintenance like water changes as part of long-term pH maintenance.
A Word of Caution
Never forget that fish adapt poorly to sudden changes in pH -- even changes toward a fish's ideal conditions. Always monitor the pH closely when you're trying to change it. If you see the pH change by more than 0.2 per day, you should stop whatever you're doing: remove driftwood or peat bags or stop adding reverse-osmosis water for the day. Excessive changes in water chemistry stress fish and can even kill them in extreme situations.