Water Quality: pH
The pH is the measure of hydrogen ion (H+) activity. More importantly for aquarists, it indicates whether water is acidic or basic. The scale for pH ranges from 1-14 with values below 7 considered acidic, values above 7 considered basic and a value of 7 being neutral.
Because pH is measured on a logarithmic scale, a one-unit change in pH is actually a ten-fold increase or decrease in hydrogen ion activity. For example, a change from a pH of 8 to a pH of 7 means that there is a ten times greater hydrogen ion activity in the solution (more hydrogen ion activity results in a lower pH). A two-unit change in pH is a 100-fold increase or decrease in hydrogen ion activity. Therefore, rapid changes of even one or two units of pH can be very detrimental to fish. Care should be taken to avoid these large changes, particularly when introducing fish into a new tank or making large water changes. Acclimate fish slowly to ensure their bodies can compensate for the pH change.
Although many fish can tolerate pH in the range of 6.5 to 8, each species of fish has an optimal pH range for growth and reproduction. Every attempt should be made to keep pH within the fish's optimal range. For this reason, mixing species of fish that have significantly differently water quality requirements is discouraged. Although pH outside of the optimum range may not kill fish, it is a constant stressor which can lower the immune system, making fish more susceptible to diseases.
The nitrifying bacteria in biofilters also have specific pH requirements. In most cases, the optimum pH for nitrification is between seven and eight although specific filters can vary. Nitrifying bacteria are inhibited below a pH of 6. If pH drops below this point, ammonia and nitrite can accumulate in the aquarium.
Carbon dioxide acts as an acid in water and, therefore, pH also exhibits a daily cycle in a planted aquarium (see Figure 1). When carbon dioxide concentrations are highest (in early morning), pH is lowest (more acidic). Throughout the day as carbon dioxide is consumed by plants for photosynthesis, pH rises (more basic). The presence of bicarbonates (alkalinity) helps buffer these daily swings in pH. See Total Alkalinity below. Alkalinity is used by the biofilter and, if not replenished through periodic water changes or chemical supplementation, can become completely depleted. If alkalinity is used up over time, the pH of water can drop drastically (often to as low as 4) in the matter of hours. This condition is known as "old tank syndrome," and fish can be killed because of the large, rapid drop in pH. Additionally, the biofilter will not be able to function at these low pHs and without bicarbonates.
The pH of the tank water should be tested once a week for optimum fish health.