Acidity and alkalinity 

The pH scale - a measure of acidity and alkalinity

The initials pH stand for "Potential of Hydrogen." Water is composed of positively charged hydrogen ions (H+) and negatively charged hydroxyl ions (OH-), the pH level is a measure of the ratio of these two ions in a body of water. Alkaline water contains a larger number of hydroxyl ions and acidic water contains more hydrogen ions. Neutral water has an equal balance of both ions. The pH scale runs from 0 (acidic) to 14 (alkaline) with 7 being described as neutral. Because the pH scale is logarithmic, a difference of one pH unit represents a tenfold, or ten times change. For example, the acidity of a sample with a pH of 6 is ten times greater than that of a sample with a pH of 7. A difference of 2 units, from 7 to 5, would mean that the acidity is one hundred times greater, and so on. Aquarium fish tend to live in water with a pH of between 5 and 9, the exact requirements will depend on which fish you intend to keep. Discus for example, are acidophiles and as such prefer acidic conditions while African cichlids, alkalophiles, prefer alkaline conditions. 

There are a number of options for testing the pH of your aquarium water ranging from test tube kits to special 'dip' sticks. However you test, the basic principles are the same. Your kit will contain the test equipment and a colour chart so that once the test is ready you can compare colours and then adjust the pH as needed. Liquids and powders have been developed to slowly raise or lower the pH level, these are available from your aquarium supplier. Special pH meters are also available, these are small hand-held devices which are dipped into the aquarium water and the pH is displayed on a digital screen.

In planted aquariums and ponds the pH level will fluctuate throughout the course of a day. This is because the plants will produce carbon dioxide at night, which lowers pH levels, and through the day they will consume CO2 by photosynthesis, which will raise pH levels. It is important to bear these fluctuations in mind when testing for pH and it is often advised to carry out tests at the same time of day each time to get as accurate a result as possible.

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