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You 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 whose balance holds the pH at the desired value.
Muriatic 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 nitrate levels rise for example. If you should decide this method to treat the water, do it BEFORE adding it to the aquarium, and use extreme caution. Acid is a dangerous chemical, especially if you don´t know what you are doing.

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.

One safe way to lower pH WITHOUT adjusting KH is to bubble CO2 (carbon dioxide) through the tank. The CO2 dissolves in water, and some of it forms carbonic acid. The formation of acid lowers the pH. Of course, in order for this approach to be practical, a steady source of CO2 bubbles is needed to hold the pH in place. As soon as the CO2 is gone, the pH bounces back to its previous value. The high cost of a CO2 injection system precludes its use as a pH lowering technique in most aquariums. CO2 injection systems are highly popular in heavily-planted tanks, because the additional CO2 stimulates plant growth.


Buffering capacity refers to water´s ability to keep the pH stable as acids or bases are added. pH and buffering capacity are intertwined with one another; although one might think that adding equal volumes of an acid and neutral water would result in a pH halfway in between, this rarely happens. If the water has sufficient buffering capacity, the buffering capacity can absorb and neutralize the added acid without significantly changing the pH. A buffer acts somewhat like a large sponge. As more acid is added, it will absorb the acid without changing the pH much. The capacity of this conceptual "sponge" is limited however; once the buffering capacity is used up, the pH changes more rapidly as acids are added.
Buffering has both positive and negative consequences. On the plus side, the nitrogen cycle produces nitric acid (nitrate). Without buffering, your tank´s pH would drop over time. With sufficient buffering, the pH stays stable. On the negative side, hard tap water often almost always has a large buffering capacity. If the pH of the water is too high for your fish, the buffering capacity makes it difficult to lower the pH to a more appropriate value.
In freshwater aquariums, most of water´s buffering capacity is due to carbonates and bicarbonates. Thus, the terms carbonate hardness (KH), alkalinity and buffering capacity are used interchangeably. Although technically not the same things, they are equivalent in practice in the context of fishkeeping. Note: the term alkalinity should not be confused with the term alkaline. Alkalinity refers to buffering, while alkaline refers to a solution that is a base.

How much buffering does your tank need? Most aquarium buffering capacity test kits actually measure KH. The larger the KH, the more resistant to pH changes your water will be. A tank´s KH should be high enough to prevent large pH swings in your tank over time. If your KH is below roughly 4.5 dH, you should pay special attention to your tank´s pH (e.g, test weekly, until you get a feel for how stable the pH is). This is ESPECIALLY important if you neglect to do frequent partial water changes. In particular, the nitrogen cycle creates a tendency for an established tank´s pH to decrease over time. The exact amount of pH change depends on the quantity and rate of nitrates produced, as well as the KH. If your pH drops more than roughly two tenths of a point over a month, you should consider increasing the KH or performing partial water changes more frequently. KH doesn´t affect fish directly, so there is no need to match fish species to a particular KH.

Note: it is not a good idea to use distilled water in your tank. By definition, distilled water has essentially no KH. That means that adding even a little bit of acid will change the pH significantly (stressing fish). Because of its instability, distilled (or any essentially pure water) is never used directly. Tap water or other salts must first be added to it in order to increase its GH and KH.