Maintaining proper pH is an important element in creating a healthy environment for your fish. Most tap water has a pH between 6.5 and 7.5. However, many of the natural chemical processes that occur in an aquarium, such as nitrification, can lower the pH. A reading below 7 is considered acidic, while readings above 7 are alkaline. There is not a single correct pH for your aquarium, but rather ranges which are appropriate for each species. Among freshwater fish, tanganyikan cichlids prefer the highest pH at 7.6 to 8.6, while discus and angelfish can be content with a pH as low as 5.0. Make sure to set your goals in accordance with the preferences of your tank inhabitants.
Test your pH. There are a wide variety of products to help you with this, from disposable pH test strips to pH meters. When selecting a pH monitor, consider how often you will be using it. In general, test strips and kits are cheaper initially, while monitors are more accurate and may save you money in the long term.
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Adjust your pH. The ease with which you can raise your pH will depend on the Kh or carbonate hardness. This is a measure of the buffering capacity of your water and can be measured with test kits that are available at your local pet store or online.
Purchase a pH increaser such as pH Up, or a pH conditioner that is rated for a specific pH. There are also products such as Seachem's Neutral Regulator that promise to bring your water to neutral (7.0), whether it started as acidic or alkaline.
If you prefer a more homegrown approach, add crushed coral, coral sand or sea shells to your tank to increase the pH while increasing the buffering capacity. You can also use baking soda to increase KH (see the calculator reference below).
Eliminate factors that may be lowering the pH in your tank. These can include tannic acid from driftwood, additional carbon dioxide (added to increase plant growth), and peat moss.