Potential of Hydrogen (pH)
The potential of hydrogen (pH) measures how acidic/basic the water is. The range goes from 0 to 14, with 7 being neutral. A pH below 7 indicates acidity, and above 7 indicates a base (basic). Hobbyists often refer to a pH above 7 as alkaline, but that term should not be confused with alkalinity, which measures buffering capacity.
pH is reported in "logarithmic units.” Each number represents a ten-fold change in the acidity/basicness of the water. Water with a pH of 6 is ten times more acidic than water having a pH of 7.
pH is affected by dissolved carbonate, bicarbonate, and gases in the water. pH is an important indicator of water that is changing chemically.
pH plays an essential role in the nitrogen cycle for systems that are maintained above a pH of 6.0. At a pH above 6.0, ammonia-oxidizing bacteria (AOB) oxidize ammonia, not ammonium. The more acidic a system gets, the greater the amount of the TAN is ammonium. The lower the pH, the greater the ratio is ammonium. If the pH is very low (at or below 6.0), there is no ammonia for AOB to process, and the ammonium/ammonia test result can be positive.
The majority of freshwater systems hobbyists keep are kept between a pH of 6.0 to 9.2. The bacteria and archaea that maintain the nitrogen cycle at a pH below 6.0 are different species. The pH of 6.0 marks a line of transition for nitrogen cycle bacteria and archaea.
When the pH drifts below a pH of 6.0 it is common to initially see a positive ammonium/ammonia test result. It can take a month or more for the bacteria and archaea that are responsible for the nitrogen cycle below 6.0 to reach a sufficient population to reduce the ammonium to 0 ppm.
Anaerobic bacteria that help reduce nitrate are also affected by low pH.
The range of the pH is directly related to carbonate hardness (KH). Plants and the nitrogen cycle use bicarbonate over time, slowly lowering the average pH of a system. Systems with very low KH (< 1 dKH) will often go acidic as the nitrate rises. Hobbyists can raise the pH by increasing the KH. Adjusting KH is covered in the water chemistry section of this ebook.
pH is inherently unstable and can have drastic swings. Day and night cycles in planted systems affect the pH. At night plants take up oxygen and give off carbon dioxide (CO2). CO2 combines with carbonate creating carbonic acid (H2CO3), which drives down the pH. During the day, the opposite happens; plants take in CO2 and give off oxygen, raising the pH.
It is common for aquarium and pond hobbyists to have a lower pH test result in the morning than in the afternoon. Having vigorous surface agitation can minimize the day and night pH fluctuation. In heavily planted systems, a drift of one full pH point or more (7.0 to 8.0) between before the lights come on and before they go off is possible. The number of plants in a system can affect the degree of day/night pH drift. Heavily planted aquariums will have the greatest pH drift.
Some aquatic plants have a common adaptation that allows them to use bicarbonate as a source of CO2. When this happens, carbonate is produced, which can radically affect pH. Vallisneria spp. are known to have a strong ability to use bicarbonate. In one of the author's test systems with Vallisneria americana (corkscrew val), the pH before the lights come on is 7.0, before the lights go off, the pH is 9.4.
Dissolved oxygen in the water will also affect the pH value. A cold water system will have a higher oxygen-carrying capacity than one kept at tropical temperatures. The increase of oxygen in cold water systems will raise the pH.
To set a baseline for the system, check the pH around the same time of day (light cycle) to eliminate the normal daily variation in results. Since pH is inherently unstable throughout 24 hours, it is better to use the KH test to measure stability. Set a KH range for the system, and the pH will be maintained around the desired range.