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What is pH and Why Is It Important?

pH is a way of measuring whether aquarium water is acidic or basic (alkaline). It is measured on a scale of 0 to 14, with 0 to 7 considered acidic and 7 to 14 considered basic or alkaline. In scientific terms, it is the ratio of hydrogen ions (H⁺) to hydroxide ions (OH⁻) that make up water molecules (H₂O). Water with a neutral pH (7.0) has equal amounts of H⁺ and OH⁻ ions, acidic water which is below 7.0, has more hydrogen ions than hydroxide ions, and basic or alkaline water, above 7.0, has more hydroxide ions than hydrogen ions. But how does that affect our fish?

aquarium pH
Check your aquarium water’s pH regularly to keep your fish, plants and other creatures healthy and happy.

Why is pH Important?

The fish, invertebrates and plants we keep in our aquariums come from environments where pH is often specific to that habitat. Maintaining the right pH in your aquarium is important to them, but it is especially vital if they were collected in the wild. In addition to requiring the correct pH and temperature range, aquatic organisms need a stable environment. Sudden changes in pH can be harmful, or even fatal, if the change is too drastic or happens suddenly. 

Does pH Change?

There is a natural tendency for pH to gradually drop in an aging aquarium as organic waste accumulates and mineral buffers are depleted. Left unchecked, pH can drop low enough to stress our aquatic pets. To avoid this, regular partial water exchanges should be done to remove pollutants and replenish minerals that naturally buffer pH and keep it stable.  

Minor changes in pH also occur between day and night, especially in well-planted tanks. Plants produce oxygen by day, which contributes to a rise in pH, and they give off carbon dioxide at night, which lowers pH. These fluctuations are more pronounced in tanks that have low buffering capacity or use supplemental CO₂.

pH is also important when cycling a new aquarium, when ammonia can build up due to a lack of nitrifying bacteria to process it. Ammonia exists as free ammonia (NH₃), which is toxic to fish, and ammonium ion (NH₄⁺), which is not as harmful. The two forms exist in equal proportions at pH of 7.3, but as pH rises above 7.3, the balance swings toward toxic NH₃. Below 7.3 the balance shifts toward non-toxic NH₄⁺. When cycling a new aquarium, test pH, ammonia and nitrite regularly to avoid stressing new fish.

aquarium pH
Maintaining proper pH is essential to a healthy aquarium.

What is the Best pH For My Fish?

There is not one pH value that suites all types of fish and other aquatic lifeforms. Since most aquarists keep fish from different environments in the same aquarium, it is best to find a pH that all tank inhabitants are comfortable at. Most freshwater tropical fish do best between 6.8 and 7.8, however, some species come from areas where pH can be significantly higher or lower than these values. For example, cichlids from the Rift Lakes of Africa are used to a pH between 8.0 and 9.0, whereas Cardinal tetras from the Rio Negro in South America live in water with a pH below 5.5! A word of caution here: most freshwater aquarium fish sold today are captive bred in water with different pH from their native environments. It is better to acclimate captive bred fish to your water chemistry than to try to match their native environment, since many of them have never been there! 

When setting up a new aquarium, test your tap water to make sure the pH and other water parameters are within the correct range for the animals and plants you plan on keeping.  Always research the water chemistry needs of fish and other aquatic creatures before buying them to make sure you have the right conditions in your aquarium.  

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