All fish have adapted to the water parameters of their home waters. This means that water that's fine for one fish can be too acidic for another. When your pH is too low -- another way of saying too acidic -- you need to figure out why and correct it to protect the health of your fish.
The Nitrogen Cycle
The nitrogen cycle is a process where bacteria eat fish waste, keeping the tank inhabitable. This process lowers pH. While this happens most dramatically when the tank is new, the process continues in established aquariums. For this reason, the pH of established aquariums tends to get lower or more acidic. Regular 25 percent water changes -- every two to four weeks -- can counteract this problem. In new aquariums, this can cause dramatic swings, while established aquariums tend to have a slow decline in pH. Test your aquarium regularly for ammonia, nitrite and pH to stay on top of these issues.
Tap Water
In some areas, the tap water has a low pH. This is uncommon, but certainly not unheard of. Additionally, well water can vary quite a bit more unpredictable than municipal tap water in terms of pH and other parameters. In order to rule this out, try testing the pH of your water straight out of the tap before adding it to your aquarium. If the water starts out acidic, you can buffer it.
Tannins
Some aquarium accessories can leach chemicals into the water, acidifying it. Tannic acids or tannins come from certain kinds of plant materials. While some fish hobbyists deliberately add driftwood or other decorative fixtures, or almond or oak leaves, to lower the pH of of their water, adding these to a tank whose fish don't prefer acidic water can have deadly consequences. You can boil driftwood to prevent it from leaching tannins.
Controlling pH
If your tank's pH is too acidic, you have several options. It's easier to lower a tank's pH than it is to raise it. Before fixing the pH, figure out the exact cause. If it's ammonia, you will have to address these underlying issues with water changes. If it's driftwood, you can boil the wood to remove tannins. From this point, you can purchase various pH buffers from pet shops. These products help raise the pH of aquarium water. Remember to raise the pH slowly, since fish react poorly to sudden changes in pH, even changes toward more ideal conditions.