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Water pH represents the concentration of hydrogen ions (H⁺) on a logarithmic scale. Because it is a logarithmic scale, a change in pH from 8 to 7 represents a 10‐fold increase in [H⁺] and from 8 to 6 represents a 100‐fold increase; small changes in pH represent large changes in [H⁺]. Water pH affects behavior, physiology, disease dynamics, drug pharmacokinetics, and sensitivity to toxins.

Frequency of testing: Assays of pH should be performed routinely; this often consists of testing the incoming water regularly or with each production batch, and testing stable systems every two weeks. Testing frequency may be increased if there is a high risk of pH fluctuations (e.g. a low alkalinity system). pH should also be assayed in transport water and prior to the addition of fish to buffered tricaine methanesulfonate (MS‐222).

Sampling: Standard sampling is described in Box A2.1. Samples should be assayed within a few hours; samples cannot be frozen and assayed later.

Testing: Colorimetric tests are common, but often have an accuracy of ±0.5 (Baird et al. 2017). Handheld pH meters that use electrometric or potentiometric methods are more accurate and readily available; they must be calibrated prior to each use.

Units: The scale ranges from 0 to 14, with no units; a pH of 7 is neutral, lower values are acidic and higher values are alkaline.

Target values: In freshwater systems, target pH may be between 5.5 and 7.5. In saltwater systems, target pH may be between 7.5 and 8.5 (Table A2.2). Some species have specific needs, e.g. African Rift Lake cichlids (Cichlidae) prefer high pH, while discus (Symphysodon spp.) and freshwater angelfish (Pterophyllum spp.) prefer low pH.

Practical considerations:

 pH needs to be within the target range for all animals in the system. While preferences are similar for most marine species, preferences of freshwater species vary widely. For example, many commonly available tropical freshwater fish are kept together at a neutral pH but prefer high pH (e.g. guppies, Poecilia reticulata) or low pH (e.g. neon tetras, Paracheirodon innesi). If a specific ecology has been targeted (e.g. Lake Victoria or Rio Negro), the life support system should be designed to replicate that pH.

 pH changes in either direction can be problematic to animals and nitrifying bacteria.

 Problems are often due to changes in buffers (such as carbonates and bicarbonates), carbon dioxide or other acids in the systems, gas exchange in the system, or the pH of the source water. New concrete can also leach materials that affect pH.

 Rapid changes are more likely to cause problems than slow changes and are more likely with low alkalinity.

 If pH increases in a system with ammonia or heavy metals, the proportion of the toxic forms increases and morbidity and mortality become more likely. This scenario is most clinically relevant with closed transport containers: when closed plastic bags are opened post‐transport, the accumulated CO2 escapes, water pH increases, and the high levels of ammonia in the transport water become more toxic. Acclimation (the slow addition of the new water) should start as soon as possible after opening closed containers.

 pH can vary significantly across the day because of diurnal changes in animal and plant respiration and photosynthesis, particularly in heavily planted systems like many outdoor ponds. pH values are lowest at dawn (Figure A2.2).

Further discussion of pH stress is available in Chapter C1.